ARM: dts: sun7i: Olimex A20-SOM-EVB: Add LRADC keys

The Olimex A20-SOM-EVB is an evaluation board for the Olimex
A20-SOM system-on-module.  It provides a set of android-style
buttons (labeled "VOL+", "VOL-", "MENU", "SEARCH", "HOME", "ESC"
and "ENTER") which are connected to a low-resolution ADC via a
resistor network.

This patch adds appropriate button definitions to the board
dts. The voltages assigned to the keys are specified in the
board schematics published by the manufacturer.

Signed-off-by: Karsten Merker <merker@debian.org>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>