arm64: mm: Temporarily disable ARM64_SW_TTBR0_PAN

We're about to rework the way ASIDs are allocated, switch_mm is
implemented and low-level kernel entry/exit is handled, so keep the
ARM64_SW_TTBR0_PAN code out of the way whilst we do the heavy lifting.

It will be re-enabled in a subsequent patch.

Bug: 69856074
Change-Id: I7c203ac936696094aa06110462d35c5c22fb42a8
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>