clk: add device tree fixed-factor-clock binding support

Add support for DT "fixed-factor-clock" binding to the common fixed
factor clock support.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Tested-by: Christian Ruppert <christian.ruppert@abilis.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>