dm: clk: Define clk_get_parent_rate() for clk operations

This commit adds the clk_get_parent_rate() function, which is responsible
for getting the rate of parent clock.
Unfortunately, u-boot's DM support for getting parent is different
(the parent relationship is in udevice) than the one in Common Clock
Framework [CCF] in Linux.

To alleviate this problem - the clk_get_parent_rate() function has been
introduced to clk-uclass.c.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>