net: dsa: qca8k: add LEDs basic support

Add LEDs basic support for qca8k Switch Family by adding basic
brightness_set() support.

Since these LEDs refelect port status, the default label is set to
":port". DT binding should describe the color and function of the
LEDs using standard LEDs api.
Each LED always have the device name as prefix. The device name is
composed from the mii bus id and the PHY addr resulting in example
names like:
- qca8k-0.0:00:amber:lan
- qca8k-0.0:00:white:lan
- qca8k-0.0:01:amber:lan
- qca8k-0.0:01:white:lan

These LEDs supports only blocking variant of the brightness_set()
function since they can sleep during access of the switch leds to set
the brightness.

While at it add to the qca8k header file each mode defined by the Switch
Documentation for future use.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>