sh: add a list of parent configurations to struct clk

Many system clocks can select a parent by writing a value to a specific field
in the configuration register. Add a list of parents and location and width of
the source selection field in the clock configuration register to struct clk to
assist in clk_set_parent() implementation.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>