[ARM] omap: create a proper tree of clocks

Traditionally, we've tracked the parent/child relationships between
clk structures by setting the child's parent member to point at the
upstream clock.  As a result, when decending the tree, we have had
to scan all clocks to find the children.

Avoid this wasteful scanning by keeping a list of the clock's children.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>