The data caches should be invalided once during startup. This should
also be done when we do not have the MMU enabled in barebox because
the Kernel does not invalidate the caches during start.

To make this sure this patch enables the arm_early_mmu_cache_invalidate
function even if MMU support is disabled. Additionally this patch adds
calls to arm_early_mmu_cache_invalidate in start.c and uncompress.c.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>