autoboot: fix bug using with CAAM and AUTOBOOT_ENCRYPTION

if  CONFIG_AUTOBOOT_KEYED, CONFIG_AUTOBOOT_ENCRYPTION and
CONFIG_AUTOBOOT_STOP_STR_SHA256 are enabled in conjunction
with CONFIG_SHA_HW_ACCEL and CONFIG_FSL_CAAM, we get the
Error when pressing a key while waiting for bootdelay:

Error: Address arguments are not aligned
CAAM was not setup properly or it is faulty

Reason is, that used variables are not cache aligned,
so malloc this variables cache aligned.

Probably this is also a bugfix for other hw accelerators
than CAAM.

Signed-off-by: Heiko Schocher <hs@denx.de>