powerpc/configs: Make pseries_le an alias for ppc64le_guest

Rather than trying to keep multiple configs up to date, make
pseries_le_defconfig an alias for ppc64le_guest_defconfig.

ppc64le_guest_defconfig should work in all cases that
pseries_le_defconfig currently does, but if not we can update it.

Move pseries_le_defconfig down in the Makefile, so it appears after
ppc64le_guest_defconfig in the help output.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230414132415.821564-31-mpe@ellerman.id.au