powerpc: Remove extern from function implementations

Sparse reports several function implementations annotated with extern.
This is clearly incorrect, likely just copied from an actual extern
declaration in another file.

Fix the sparse warnings by removing extern.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231011053711.93427-6-bgray@linux.ibm.com