clk: mvebu: add clock gating control provider for DT

This driver allows to provide DT clocks for clock gates found on
Marvell Dove and Kirkwood SoCs. The clock gates are referenced by
the phandle index of the corresponding bit in the clock gating control
register to ease lookup in the datasheet.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>