[MTD] PMC MSP71xx flash/rootfs mappings

Add flash and rootfs mappings for the PMC-Sierra MSP71xx devices.

This patch references some platform support files previously submitted to
the linux-mips@linux-mips.org list.

Signed-off-by: Marc St-Jean <Marc_St-Jean@pmc-sierra.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>