Several clock API functions handle NULL clocks when the Common Clock
Framework is used, while their legacy SH counterparts don't, and would
just crash when a NULL clock is passed.

Add NULL checks to clk_get_rate(), clk_set_rate(), clk_get_parent(), and
clk_round_rate(), to avoid different behavior in drivers shared between
legacy and CCF-based platforms.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>