Create a RouterBoard platform and use it to create a flash map

Summary:
The RouterBoard uses a predefined partition map which doesn't exist in the fdt.
This change allows overriding the fdt slicer with a custom slicer, and uses this
custom slicer to define the flash map on the RouterBoard RB800.
D3305 converts the mpc85xx platform into a base class, so that systems based on
the mpc85xx platform can add their own overrides. This change builds on D3305,
and creates a RouterBoard (RB800) platform to initialize the slicer override.

Reviewed By: nwhitehorn, imp
Differential Revision: https://reviews.freebsd.org/D3345