arm64: fix comment above tcr_compute_pa_size

The 'pos' argument is used to select where in TCR to write the value:
the IPS or PS bitfield.

Fixes: 787fd1d0

 ("arm64: limit PA size to supported range")
Signed-off-by: Kristina Martsenko <kristina.martsenko@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>