powerpc/85xx: Refactor some defines out of corenet_ds.h

Move some SoC/board specific defines out of corenet_ds.h and into the
corresponding P3041DS/P4080DS/P5020.h.

We moved CONFIG_MMC, CONFIG_PCIE3, & CONFIG_FSL_NGPIXIS because the P3060
SoC/reference board does not have these devices and it will share the same
board code.

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