dts: bindings: lpadc: Add regulator phandle prop

Add phandle prop to reference any regulator that must
be enabled in order for the LPADC to function as intended.

Change LPADC driver to use this property if present.

LPADC on LPC55S36 depends on VREF peripheral, enable for this platform.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>