armv7: cpu_init_cp15: invalidate L1 data cache

For at least an mx6q machine, if
u-boot is started by ROM usb loader and
CONFIG_SYS_DCACHE_OFF is defined, then
dcache will contain garbage that will
prevent Linux from decompressing.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
mx6: disable L2 cache in lowlevel_init
mx6h: squash