powerpc/32s: Allow deselecting CONFIG_PPC_FPU on mpc832x

The e300c2 core which is embedded in mpc832x CPU doesn't have
an FPU.

Make it possible to not select CONFIG_PPC_FPU when building a
kernel dedicated to that target.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/fcdc60d85baf80eaa0a7f3261d9d889282068216.1597770847.git.christophe.leroy@csgroup.eu