powerpc/Kconfig: Fix non existing CONFIG_PPC_FSL_BOOKE

CONFIG_PPC_FSL_BOOKE doesn't exist. Should be CONFIG_FSL_BOOKE.

Fixes: 49e3d8ea ("powerpc/fsl_booke: Enable STRICT_KERNEL_RWX")
Cc: stable@vger.kernel.org
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/828f6a64eeb51ce9abfa1d4e84c521a02fecebb8.1663606875.git.christophe.leroy@csgroup.eu