powerpc/64: Use {SAVE,REST}_NVGPRS macros

In entry_64.S there are places that open code saving and restoring the
non-volatile registers. There are already macros for doing this so use
them.

Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
Reviewed-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191211023552.16480-1-jniethe5@gmail.com