[ARM] MV78XX0: MPP routines and definitions

[ARM] MV78XX0: MPP routines and definitions

This patch is composed of two new files :

- mpp.c which is mainly inspired by the same file as in mach-kirkwood

- mpp.h that is written from the documentation provided by Marvell
http://www.marvell.com/products/processors/embedded/discovery_innovation/HW_MV78100_OpenSource.pdf

Moreover, due to some implementation problem, I have
defined some MPPX_UNUSED that offer developers the possibility
to SET MPP to some unused value (such as for Buffalo WXL).

Note: This patch doesn't support MV78200 yet (only 78100 MPP lines have
been written)

Signed-off-by: Sebastien Requiem <sebastien@kolios.dk>
Signed-off-by: Nicolas Pitre <nico@marvell.com>