ARM: shmobile: r8a7793: Add GPIO nodes to device tree

Add r8a7793 GPIO device nodes that are assumed to be identical
to r8a7791. This matches the data sheet for GPIO and MSTP bits.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>