dt-bindings: clk: document PolarFire SoC fabric clocks

On PolarFire SoC there are 4 PLL/DLL blocks, located in each of the
ordinal corners of the chip, which our documentation refers to as
"Clock Conditioning Circuitry". PolarFire SoC is an FPGA, these are
highly configurable & many of the input clocks are optional.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220908143651.1252601-3-conor.dooley@microchip.com