ARM: dts: silk: Add GPIO keys to DT

Extend the Silk board support to include SW3, SW4, SW6 and SW12. They
are all connected via GPIO lines and handled by the gpio-keys driver.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>