clk: Add clock driver for AXM55xx SoC

Add clk driver to support clock blocks found on the AXM55xx devices. The driver
provides clock implementations for three different types of clock devices on
the AXM55xx device: PLL clock, a clock divider and a clock mux.

Signed-off-by: Anders Berg <anders.berg@lsi.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>