powerpc: simplify patch_instruction_site() and patch_branch_site()

Using patch_site_addr() helper, patch_instruction_site() and
patch_branch_site() can be simplified and inlined.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>