arm64: dts: renesas: salvator-common: Add GPIO keys support

The Salvator-X and XS boards have a 4 lines DIP switch and 3 push
buttons connected to SoC GPIOs, meant to be used as general-purpose test
keys. Add a corresponding node in DT, mapping (semi-randomly) the DIP
switch to keys 1-4 and the push buttons to keys A-C.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>