clk: samsung: pll: Add support for rate configuration of PLL46xx

This patch implements round_rate and set_rate callbacks of PLL46xx
driver to allow reconfiguration of PLL at runtime.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>