dm: clk: Extend clk_get_parent_rate() to support CLK_GET_RATE_NOCACHE flag

If the CLK_GET_RATE_NOCACHE flag is set - the clk_get_parent_rate()
provides recalculated clock value without considering the cache setting.

This may be necessary for some clocks tightly coupled with power domains
(i.e. imx8), and prevents from reading invalid cached values.

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