arm64: dts: freescale: imx93-phyboard-nash: Move ADC vref to SoM

arm64: dts: freescale: imx93-phyboard-nash: Move ADC vref to SoM

Move configuration for ADC voltage reference from board DTS to a SoM
include file. The SoC ADC reference voltage is connected to a "VDDA_1V8"
voltage node and supplied by the PMIC's BUCK5 regulator. The reference
voltage is thus defined by the SoM and cannot be changed by the carrier
board design and as such belongs into the SoM include file.

Moreover, with this in place, customers designing own carrier boards can
simply include imx93-phycore-som.dtsi and enable adc1 in their own DTS
without the need to define dummy ADC vref regulator themselves anymore.

Signed-off-by: Primoz Fiser <primoz.fiser@norik.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>