clk: uniphier: add CPU-gear change (cpufreq) support

Core support code for CPU frequency changes, which will be used by
the generic cpufreq driver.

The register view is different from the generic clk-mux; it has
a separate status register, and an update bit to load the register
setting.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>