arm64: Disable TTBR1 maps in EL1

When running in EL1, AArch64 knows two page table maps. One with addresses
that start with all zeros (TTBR0) and one with addresses that start with all
ones (TTBR1).

In U-Boot we don't care about the high up maps, so just disable them to ensure
we don't walk an invalid page table by accident.

Reported-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>