clk: uniphier: rework for better clock tree structure

clk: uniphier: rework for better clock tree structure

U-Boot does not support fancy clock tree structures like the Linux
common clock framework.  Implement a simple clock tree model at the
driver level.  With this, the clock data will be simplified.

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