clk: microchip: add PolarFire SoC fabric clock support

clk: microchip: add PolarFire SoC fabric clock support

Add a driver to support the PLLs in PolarFire SoC's Clock Conditioning
Circuitry, an instance of which is located in each ordinal corner of
the FPGA. Only get_rate() is supported as these clocks are intended to
be statically configured by the FPGA design. Currently, the DLLs are
not supported by this driver. For more information on the hardware, see
"PolarFire SoC FPGA Clocking Resources" in the link below.

Link: https://onlinedocs.microchip.com/pr/GUID-8F0CC4C0-0317-4262-89CA-CE7773ED1931-en-US-1/index.html
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220908143651.1252601-5-conor.dooley@microchip.com