qemu-arm: Drop ARCH_SUPPORT_TFABOOT

CONFIG_ARCH_SUPPORT_TFABOOT was used on the qemu-arm64 platform to
guard a tweak to the flash bank configuration. U-Boot now reads the
current flash setup from the devicetree, so there is no need for
this option anymore.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Tom Rini <trini@konsulko.com>