ARM: shmobile: Enable fixed voltage regulator in shmobile_defconfig

This is selected by MACH_MARZEN which is enabled in shmobile_defconfig,
however, MACH_MARZEN is going away along with the board code it enables.

Acked-by: Magnus Damm <damm@opensource.se>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>