sh: clkfwk: Improve the generic clk_set_parent() implementation.

This causes the generic clk_set_parent() implementation to be a bit more
intelligent. A clk_reparent() is added to move the clock over to the new
parent's sibling list, which then allows the generic rate propagation
code to succeed. This also becomes a nop if the new and old parents are
unchanged.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>