arm-bsp: musca-b1: Add Zephyr support

Zephyr supports Musca B1, and QEMU has an emulated Musca B1 board.  Add
the relevant parts to get this compiling with meta-zephyr and (in
theory) run QEMU.

Change-Id: I9af4aea03e31e96f8eee17f44913e747b1241891
Signed-off-by: Jon Mason <jon.mason@arm.com>