bootm: Align cache flush begin address

commit b4d956f6 ("bootm: Align cache flush end address correctly")
aligns the end address of the cache flush operation to a cache-line size to
ensure lower-layers in the code accept the range provided and flush.

A similar action should be taken for the begin address of a cache flush
operation. The load address may not be aligned to a cache-line boundary, so
ensure the passed address is aligned.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reported-by: Breno Matheus Lima <brenomatheus@gmail.com>
Suggested-by: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
Tested-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>