mfd: asic3: add clock handling for MFD cells

Since ASIC3 has to work on both PXA and S3C and since their
struct clk implementations differ, we can't register out
clocks with the clkdev mechanism (yet?).
For now we have to keep clock handling internal to this
driver and enable/disable the clocks via the
mfd_cell->enable/disable functions.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>