powerpc: Add allmodconfig for all 32-bit sub-arches

32-bit powerpc kernels can be built for one of 5 sub-arches, see
Kconfig.cputype:

  PPC_BOOK3S_32: "512x/52xx/6xx/7xx/74xx/82xx/83xx/86xx"
  PPC_85xx: "Freescale 85xx"
  PPC_8xx: "Freescale 8xx"
  40x: "AMCC 40x"
  44x: "AMCC 44x, 46x or 47x"

By default none of these are built for a plain allmodconfig build,
because it selects PPC64 which builds a 64-bit kernel.

There is already a ppc32_allmodconfig, which enables PPC_BOOK3S_32.

Add similar targets for the other 32-bit sub-arches to increase build
coverage:
  ppc40x_allmodconfig
  ppc44x_allmodconfig
  ppc8xx_allmodconfig
  ppc85xx_allmodconfig

Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240229114108.743810-1-mpe@ellerman.id.au