dm: clk: Define clk_get_parent() for clk operations

This commit adds the clk_get_parent() function, which is responsible
for getting the parent's struct clock pointer.

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 obtain the pointer to struct clk of parent the
pdev->uclass_priv field is read via dev_get_clk_ptr() wrapper.

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