mediatek: fix pwn fan settings for sinovoip bpi-r3 (v2)

Popular bpi-r3 pwm fans like this one

  https://www.amazon.com/youyeetoo-Barebone-Fan-BPI-R3-Integrated/dp/B0CCCTY8PS

will not work properly with current openwrt-23.05/24.10/upstream firmwares.
Trying different pwm setting

  echo $value > /sys/devices/platform/pwm-fan/hwmon/hwmon1/pwm1

I found:

  pwm1 value     fan rotation speed   cpu temperature     notes
  -----------------------------------------------------------------
    0            maximal              31.5 Celsius        too noisy
   40            optimal              35.2 Celsius        no noise hearable
   95            minimal
   above 95      does not rotate      55.5 Celsius
  -----------------------------------------------------------------

At the moment we have following cooling levels:

  cooling-levels = <255 96 0>;

for cpu-active-low, cpu-active-medium and cpu-active-high modes correspondingly.
Thus only cpu-active-high and cpu-active-low are usable. I think this is wrong.

This patch fixes cpu-active-medium settings for bpi-r3 board.

PS: I know, the patch is not ideal as it can break pwm fan for some users.
    There are some peoples that use handmade cooling solutions, but:
      * discussed cooler is the only 'official' pwm cooler for bpi-r3
        available on the market.
      * most peoples will use passive cooling available on the market or
        the discussed cooler.
      * the pwm-fan dts section was added before the official cooler
	appears on the market.
    Thus it should not be a lot of harm from this fix.

This patch may not be enough, users may wants to tweak their thermal_zone0
trip points, thus tuning fan rotation speed depending on cpu temperature.
That can be done by editing /etc/rc.local file like this:

  === /etc/rc.local ===
  # Put your custom commands here that should be executed once
  # the system init finished. By default this file does nothing.

  # cpu temterature below 25 Celsius degrees, no rotation
  echo 25000 > /sys/class/thermal/thermal_zone0/trip_point_4_temp
  # cpu temperature in [25..32] Celsius degrees, normal rotation speed
  echo 32000 > /sys/class/thermal/thermal_zone0/trip_point_3_temp
  # cpu temperature above 50 Celsius degrees, max rotation speed
  echo 50000 > /sys/class/thermal/thermal_zone0/trip_point_2_temp

  exit 0
  =====================

Changes v2:
* put patch to a proper directory
* updated patch description
* tested with latest openwrt based on linux-6.6

Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@gmail.com>

target/linux/mediatek/patches-6.6/196-dts-mt7986a-bpi-r3-pwm-fan-cooling-levels.patch

@@ -0,0 +1,11 @@
+--- a/arch/arm64/boot/dts/mediatek/mt7986a-bananapi-bpi-r3.dts
++++ b/arch/arm64/boot/dts/mediatek/mt7986a-bananapi-bpi-r3.dts
+@@ -46,7 +46,7 @@
+ 		compatible = "pwm-fan";
+ 		#cooling-cells = <2>;
+ 		/* cooling level (0, 1, 2) - pwm inverted */
+-		cooling-levels = <255 96 0>;
++		cooling-levels = <255 40 0>;
+ 		pwms = <&pwm 0 10000>;
+ 		status = "okay";
+ 	};