drivers: sh: clk: Remove obsolete and unused clk_round_parent()

clk_round_parent() was only ever used by AP4EVB, until commit
b24bd7e9 ("ARM: shmobile: Remove AP4EVB board support").

The Common Clock Framework does not provide clk_round_parent(), hence
remove it.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>