[POWERPC] 85xx: rework platform Kconfig

* Allow multiple boards to be selected in a single build
* Removed Kconfig option '85xx' which existed only for compat with arch/ppc
* Added a multiplatform 85xx defconfig (mpc85xx_defconfig). This builds
  all 85xx boards except sbc8560 and stx_gp3 since these to boards have
  board specific ifdef in driver code that may break all other boards

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>