boston: Introduce support for the MIPS Boston development board

This patch introduces support for building U-Boot to run on the MIPS
Boston development board. This is a board built around an FPGA & an
Intel EG20T Platform Controller Hub, used largely as part of the
development of new CPUs and their software support. It is essentially
the successor to the older MIPS Malta board.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>