ARM64: dts: meson-gxm: add the PHY interrupt line on Khadas VIM2

The INTB/PMEB pin of the RTL8211F PHY on the Khadas VIM2 is routed to
GPIOZ_15. Add the corresponding interrupt using the GPIO interrupt
controller so the PHY framework doesn't have to poll the PHY for it's
status.

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