ARM: shmobile: Enable HIGHMEM in shmobile_defconfig

Many mach-shmobile hardware platforms include support for more
than 1GiB of RAM. Enable HIGHMEM by default to allow use of
larger amounts of memory.

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