ARM: shmobile: defconfig: enable initrd

Enable initrd support.

Signed-off-by: Kevin Hilman <khilman@linaro.org>
[horms+renesas@verge.net.au: dropped enabling atag dtb compat]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Merge branch 'heads/defconfig-for-v3.18' into next