Add driver for Renesas 9-series PCIe clock generators. This driver
is designed to support 9FGV/9DBV/9DMV/9FGL/9DML/9QXL/9SQ series I2C
PCIe clock generators, currently the only tested and supported chip
is 9FGV0241.

The driver is capable of configuring per-chip spread spectrum mode
and output amplitude, as well as per-output slew rate.
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: devicetree@vger.kernel.org
Link: https://lore.kernel.org/r/20220226040723.143705-3-marex@denx.de
[sboyd@kernel.org: Use non-underscore API for fixed factor]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>