armv8: Fix TCR 64-bit writes

The AArch64 TCR_ELx register is a 64-bit register, and many newer
architecture features use bits in the upper half. So far U-Boot was
igorant of those bits, trying to leave them alone.
However, in an effort to set bit 31 to 1, it failed doing so, because
the compiler sign-extended "1 << 31", so that all bits[63:31] got set.

Older ARMv8.0 cores don't define anything dangerous up there, but newer
architecture revisions do, and setting all those bits will end badly:
=================
$ qemu-system-aarch64 -cpu max ....
U-Boot 2022.07-rc1 (May 09 2022 - 15:21:00 +0100)

DRAM:  1.5 GiB
=================  (hangs here)

Defining TCR_ELx_RSVD to "1U << 31" avoids the sign-extension, so all
upper bits stay at a safe 0 value. This means no more surprises when
U-Boot runs on a more capable CPU core.

Reported-by: Balaji Anandapadmanaban <Balaji.Anandapadmanaban@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Tested-by: Peter Collingbourne <pcc@google.com>
Reviewed-by: Peter Collingbourne <pcc@google.com>