clk: mvebu: powersave clock is a multiplexer

Kirkwood is unique among the mvebu SoCs for having a clock multiplexer
that feeds into the cpu. This multiplexer can select either the cpu pll
or the ddr clock as its input signal, allowing for a choice between
performance and power savings.

This patch introduces the code needed to register the clock multiplexer
on Kirkwood SoCs but does not include the clock data to actually
register the clock. That will be done in a follow-up patch which is
necessary to prevent breaking git bisect.

Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Mike Turquette <mturquette@linaro.org>