ARM: mach-shmobile: mackerel: enable REGULATOR in defconfig

As well as being a generally sane thing to do this
is required for MMCIF to function in conjunction with
" ARM: shmobile: streamline mackerel SD and MMC devices".

Cc: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>