clk: mvebu: add common clock functions for core clk and clk gating

Based on the current common functions for core clocks and clock
gating control, new common functions are joined in a single file.
Given the opportunity, names of functions and structs are unified,
and also a Kconfig entry is added.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>