powerpc/configs: Make pseries_defconfig an alias for ppc64le_guest

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

NOTE, pseries_defconfig was a big endian config, but this commit
switches it to little endian.

Almost all distros are ppc64le these days, so little endian is much more
likely to be what a user wants when they build for "pseries".

For an actual big endian guest, use ppc64_guest_defconfig.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230414132415.821564-32-mpe@ellerman.id.au