powerpc/amigaone: Make amigaone_discover_phbs() static

It's only used in setup.c, so make it static.

Fixes: 053d58c8

 ("powerpc/amigaone: Move PHB discovery")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210210130804.3190952-3-mpe@ellerman.id.au