powerpc/85xx: split sbc8548 dts file into pre and post chunks

powerpc/85xx: split sbc8548 dts file into pre and post chunks

Updates to u-boot allow this board to boot off of either
the 8MB soldered on flash, or the 64MB SODIMM flash.

This is achieved by changing JP12 and SW2.8 which in turn
swaps which flash device appears on /CS0 and /CS6 respectively.

Since the flash devices are not the same size, this also
changes the MTD memory map layout on the local bus.

Here we split the common chunks out into a pre and post
include, so they can be reused by an upcoming "alternative
boot" dts file; leaving only the local bus chunk behind.

No content changes are made at this point - it is just purely
the move to using include files.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>