kirkwood: add save functionality kirkwood_mpp_conf function

kirkwood: add save functionality kirkwood_mpp_conf function

If a second non NULL argument is given to the kirkwood_mpp_conf
function, it will be used to store the current configuration of the MPP
registers. mpp_save  must be a preallocated table of the same size as
mpp_list and it must be zero terminated as well.

A later call to kirkwood_mpp_conf function with this saved list as first
(mpp_conf) argment will set the configuration back.

Signed-off-by: Valentin Longchamp <[email protected]>
cc: Holger Brunck <[email protected]>
cc: Prafulla Wadaskar <[email protected]>