As the mvebu_mpp_ctrl structures contain function pointers, it is
preferable for these to be made read-only to prevent the function
pointers being modified.  So make these const.
Signed-off-by: NRussell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>