ARM: shmobile: defconfig: Disable CONFIG_FB_SH_MOBILE_MERAM

The last Renesas ARM platform using this driver was removed in commit
a521422e ("ARM: shmobile: mackerel: Remove Legacy C board
code").

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