clk: meson: mpll: add round closest support

Allow the mpll driver to round the requested rate up if
CLK_MESON_MPLL_ROUND_CLOSEST is set and it provides a rate closer to the
requested rate.

Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Martin <Blumenstingl&lt;martin.blumenstingl@googlemail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>