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: 787fd1d019b2 ("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>

arch/arm64/include/asm/assembler.h

@@ -360,7 +360,7 @@ alternative_endif
  * ID_AA64MMFR0_EL1.PARange value
  *
  *	tcr:		register with the TCR_ELx value to be updated
- *	pos:		PARange bitfield position
+ *	pos:		IPS or PS bitfield position
  *	tmp{0,1}:	temporary registers
  */
 	.macro	tcr_compute_pa_size, tcr, pos, tmp0, tmp1