powerpc/44x: Add PowerPC 44x simple platform support

This adds a common board file for almost all of the "simple" PowerPC 44x
boards that exist today.  This is intended to be a single place to add
support for boards that do not differ in platform support from most of the
evaluation boards that are used as reference platforms.  Boards that have
specific requirements or custom hardware setup should still have their own
board.c file.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>