ARM: dts: rockchip: add adc-keys node to rk3066a-mk808

ARM: dts: rockchip: add adc-keys node to rk3066a-mk808

The MK808 has a button inside the cover for the boot loader to do
some action. Add the adc-keys node to the rk3066a-mk808.dts file.
The rk3066 has a higher maximum DC supply voltage for the analog part of
SAR-ADC VDDA_SARADC of 2.75V then other Rockchip SoCs.
For the "rockchip,saradc" node is a vref-supply property required,
so add a regulator for it as well.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Link: https://lore.kernel.org/r/20210828092755.24560-1-jbx6244@gmail.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>