clk: samsung: Remove unimplemented ops for pll

Unimplemented clock operations should be simply omitted instead of returning
error values.

This patch removes unimplemented PLL operations to fix problems caused
by returning error code in round_rate callback.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Reviewed-by: Thomas Abraham <thomas.abraham@linaro.org>
Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>