powerpc: Add ppc32_allmodconfig defconfig target

Because the allmodconfig logic just sets every symbol to M or Y, it
has the effect of always generating a 64-bit config, because
CONFIG_PPC64 becomes Y.

So to make it easier for folks to test 32-bit code, provide a phony
defconfig target that generates a 32-bit allmodconfig.

The 32-bit port has several mutually exclusive CPU types, we choose
the Book3S variants as that's what the help text in Kconfig says is
most common.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>