treewide: use CONFIG_IS_ENABLED() for ARMV8_SEC_FIRMWARE_SUPPORT

There is SPL_ARMV8_SEC_FIRMWARE_SUPPORT and ARMV8_SEC_FIRMWARE_SUPPORT.
Thus use CONFIG_IS_ENABLED() instead of the simple #ifdef.

Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Michal Simek <michal.simek@xilinx.com>