clk: add MOXA ART SoCs clock driver

MOXA ART SoCs allow to determine PLL output and APB frequencies
by reading registers holding multiplier and divisor information.

Add a clock driver for this SoC.

Signed-off-by: Jonas Jensen <jonas.jensen@gmail.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>