powerpc/Makefile: Add generated defconfigs to help output

Currently none of the generated defconfigs appear in the help output,
because the help text discovers defconfigs by looking for actual files
named "*_defconfig".

Collect the generated defconfig names into a variable and then print
those out in archhelp.

Output looks like eg:

  pseries_le_defconfig        - Build for pseries_le
  ppc64le_defconfig           - Build for ppc64le
  ppc64le_guest_defconfig     - Build for ppc64le_guest
  ...
  ppc64_randconfig            - Build for ppc64_randconfig

  adder875_defconfig          - Build for adder875
  amigaone_defconfig          - Build for amigaone

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
[mpe: Fix PHONY bug which broke in-tree build, thanks rmclure]
Link: https://msgid.link/20230329072334.2023357-2-mpe@ellerman.id.au