Sigma-delta modulation is supported for some PLLs. This allows
fractional-N multipliers to be used. In reality we don't know
how to configure the individual settings for it. However we can
copy existing settings from the vendor kernel to support clock
rates that cannot be generated from integer factors, but are
really desired. The vendor kernel only uses this for the audio
PLL clock, and only on the latest chips.

This patch adds a new class of clocks, along with helper functions.
It is intended to be merged into N-M-factor style clocks as a
feature, much like fractional clocks.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>