ARM: dts: wheat: add support for tactile switches

Add  support for the  tactile switches  SW2/3 (on the debug board) as a
virtual keyboard like it  was done with the Blanche board).

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>