powerpc: Fix 85xx build

The merge of the kbuild tree dropped the renaming of the FSL_BOOKE
kconfig option.

Fixes: 8afc66e8

 ("Merge tag 'kbuild-v6.1' of git://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild")
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>