clk: lochnagar: Use new parent_data approach to register clock parents

Switch over to the more modern style of registering parents and simplify
the code in the process.

Suggested-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>