microblaze: Remove zalloc_maybe_bootmem()

Last user of zalloc_maybe_bootmem() was removed by
commit 4308e3c9 ("microblaze/PCI: Remove unused allocation & free
of PCI host bridge structure")

Remove it.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Link: https://lore.kernel.org/r/bfb1601cac24d7ef8e741c83f9301e4a5a87a0b5.1692124505.git.christophe.leroy@csgroup.eu


Signed-off-by: Michal Simek <michal.simek@amd.com>