dt-bindings: fpga: add binding doc for microchip-spi fpga mgr

Add Device Tree Binding doc for Microchip Polarfire FPGA Manager using
slave SPI to load .dat formatted bitstream image.

Signed-off-by: Ivan Bornyakov <i.bornyakov@metrotek.ru>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Xu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/20220623163248.3672-5-i.bornyakov@metrotek.ru


Signed-off-by: Xu Yilun <yilun.xu@intel.com>