ARM: shmobile: Kconfig: remove obsolete option ARCH_R8A7778

Obsoleted by removal of r8a7778 legacy platform.

Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Acked-by: by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>