ARM: shmobile: Move rcar-gen2.h, cleanup r8a7790 case

Change location of rcar-gen2.h so it can be used as #include "rcar-gen2.h"
instead of the old style #include <mach/rcar-gen2.h>. Also clean up
the r8a7790 case to follow the same style as r8a7791.

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