ARM: shmobile: Remove mach/system.h

The file <mach/system.h> is no longer included from anywhere so
clean up the code by removing mach-shmobile/include/mach/system.h

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>