arm64: dts: rockchip: Add support for volume keys to rk3399-pinephone-pro

arm64: dts: rockchip: Add support for volume keys to rk3399-pinephone-pro

These are implemented via regular ADC, so regular polling is needed,
for these keys to work.

Co-developed-by: Martijn Braam <martijn@brixit.nl>
Signed-off-by: Martijn Braam <martijn@brixit.nl>
Co-developed-by: Kamil Trzciński <ayufan@ayufan.eu>
Signed-off-by: Kamil Trzciński <ayufan@ayufan.eu>
Signed-off-by: Ondrej Jirman <megi@xff.cz>
Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Tested-by: Ondrej Jirman <megi@xff.cz>
Link: https://lore.kernel.org/r/20230405123813.2272919-1-pbrobinson@gmail.com
[increased Volume-Down voltage to 600mV as suggested by Ondrej]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>