arm64: Move PSTATE.TCO setting to separate functions

For consistency with __uaccess_{disable,enable}_hw_pan(), move the
PSTATE.TCO setting into dedicated __uaccess_{disable,enable}_tco()
functions.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>