ramips: ethernet: ralink: add led_source dts-binding

ramips: ethernet: ralink: add led_source dts-binding

this adds the new dts-binding "mediatek,led_source"
currently for MT7628AN and MT7688 built-in switches,
which is documented as a 3-bit field configuring the
switch LEDs for various control schemes from 0 to 3.

Normally this is not needed, but e.g. for Asus RT-AC1200-V2
it is a must to set it to the anyway undocumented value
of 4, to have the switch LEDs react correctly on link/act
events. This is an MT7628DAN device, but I doubt this is
a speciality of this particular SoC.

Also added the RT305X_ESW_LED_OFF value to LED states.
Did also rename the register RT5350_EWS_REG_LED_POLARITY
to RT5350_EWS_REG_LED_CONTROL, which is the correct name.
Also making use of defines for some hardcoded values.

Signed-off-by: Tamas Balogh <tamasbalogh@hotmail.com>