selftests/powerpc: Use mfspr/mtspr macros

No need to write inline asm for mtspr/mfspr, we have macros for this
in reg.h

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Reviewed-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221128041948.58339-2-bgray@linux.ibm.com