ARM: 5697/1: MMCI Break out clock divider setup

This breaks out the clock divider set-up code from the
mmci_set_ios() code and surrounds the two register
writes with a host lock so we don't get collisions if
(in future code) two code paths want to change the
clock divider at the same time as can be the case if
we get something like pre/post- clock frequency change
notifications soonish.

Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>