ARM: dts: sun8i: tbs-a711: Add support for volume keys input

TBS A711 tablet has volume up/down keys connected to r_lradc. Add
support for these keys.

Signed-off-by: Ondrej Jirman <megous@megous.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>