board: rockchip: Add early ADC button detect for RGxx3

Add ADC button detect for early SPL stage for RGxx3 device. This is
important because on at least the RG353P and RG353V a clk pin is not
exposed that would allow us to take the eMMC out of the boot path.

Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>