clk: meson: mpll: properly handle spread spectrum

The bit 'SSEN' available on some MPLL DSS outputs is not related to the
fractional part of the divider but to the function called
'Spread Spectrum'.

This function might be used to solve EM issues by adding a jitter on
clock signal. This widens the signal spectrum and weakens the peaks in it.

While spread spectrum might be useful for some application, it is
problematic for others, such as audio.

This patch introduce a new flag to the MPLL driver to enable (or not) the
spread spectrum function.

Fixes: 1f737ffa ("clk: meson: mpll: fix mpll0 fractional part ignored")
Tested-by: Martin <Blumenstingl&lt;martin.blumenstingl@googlemail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>