rk3399 ATF requires arm toolchain to build the m0 pmu driver.
As OpenWrt doesn't ship this toolchain so download the prebuilt one
just like what we did in arm-trusted-firmware-mvebu.
Fixes: 5d1cb52da062 ("arm-trusted-firmware-rockchip: Update to 2.9")
Reported-by: Wurzer Juergen <wurzer.juergen@gmail.com>
Signed-off-by: Tianling Shen <cnsztl@immortalwrt.org>