drivers: sh: clk: Avoid crashes when passing NULL clocks

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>