ARM: shmobile: Add DTS gpio-keys support for SW2 on Koelsch

Add DTS gpio-keys support for SW2 on the Koelsch board.
This makes the DT code match the legacy board code. Also
update the existing gpio-keys nodes to make use of KEY_n.

Signed-off-by: Magnus Damm <damm@opensource.se>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>