imx8m{m,n}-venice-gw7902: add GSC ADC rail for VDD_5P0

The GW7902-C revision adds an ADC for the VDD_5P0 voltage rail.
Add register definitions for it.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Acked-by: Peng Fan <peng.fan@nxp.com>