powerpc: Drop zalloc_maybe_bootmem()

The only callers of zalloc_maybe_bootmem() are PCI setup routines. These
used to be called early during boot before slab setup, and also during
runtime due to hotplug.

But commit 5537fcb3 ("powerpc/pci: Add ppc_md.discover_phbs()")
moved the boot-time calls later, after slab setup, meaning there's no
longer any need for zalloc_maybe_bootmem(), kzalloc() can be used in all
cases.

Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230823055430.752550-1-mpe@ellerman.id.au