arm: v7: Kconfig: Add entry for MMU

Add a Kconfig entry for MMU and imply for all platforms using
cache-cp15.c containing MMU setup. Using imply instead of select so that
MMU can be disabled by defconfigs when not needed.

Tested-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>