[powerpc] Tighten contraints for asm constant parameters

There are a few places where only known numeric values are acceptable for
`asm` parameters, yet the constraint "i" is used.  "i" can include
"symbolic constants whose values will be known only at assembly time or
later."

Use "n" instead of "i" where known numeric values are required.

Suggested-by: Segher Boessenkool <segher@kernel.crashing.org>
Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>