powerpc: Fix 85xx build
The merge of the kbuild tree dropped the renaming of the FSL_BOOKE
kconfig option.
Fixes: 8afc66e8d43b ("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>
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