mediatek: dts: mt7988a: wire-up mediatek,pio for PHY LEDs

mediatek: dts: mt7988a: wire-up mediatek,pio for PHY LEDs

The PHY driver needs to read a register containing the values of the
bootstrap pins (which happen to be the PHY LEDs) to determine the LED
polarities. Allow regmap access to first pinctrl bank by adding the
'syscon' compatible, and reference the pinctrl in the MDIO bus where
the PHY driver will look for it.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
(cherry picked from commit 1f1e0b1144ebaa4ba8b948a12d989a0a6fc9b76f)