ARM64: dts: meson: add comments with the GPIO for the PHY interrupts

Currently one has to look/calculate the GPIO for the PHY interrupts
manually. Add a comment for the existing PHY interrupt lines to make it
easier to find out which GPIO is used.
This is done using the following calculation:
- number of GPIO AO pins (14 on GXBB: GPIOAO_0..13)
- add the offset of the pin which is used for the interrupt (for example
  GPIOZ_15 = 15 on Odroid-C2)

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-By: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>