Make the mpc85xx platform a kobj base class.

Summary:
Some systems are based around mpc85xx, but need special initialization. By
making the mpc85xx platform a base class, these systems can be platform
subclasses, and perform board-specific initialization in addition to the mpc85xx
initialization.

Test Plan:
Tested on my RB800. A platform class was created, and will be committed
separately.

Reviewed By: nwhitehorn
Differential Revision: https://reviews.freebsd.org/D3305