clk: uniphier: rework UniPhier clk driver

The initial design of the UniPhier clk driver for U-Boot was not
very nice.  Here is a re-work to sync it with Linux's clk and reset
drivers, maximizing the code reuse from Linux's clk data.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>