Without this change, internal and external B53 switches couldn't coexist as
reported in https://github.com/openwrt/openwrt/issues/10313.
In order to fix this we need to force the B53 MMAP DSA switch driver to use
bcm6368-mdio-mux for accessing the PHY registers instead of its own phy_read()
and phy_write() functions.
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
