dt-bindings: arm: arm,juno-fpga-apb-regs: document FPGA syscon

dt-bindings: arm: arm,juno-fpga-apb-regs: document FPGA syscon

Add dedicated bindings for the FPGA syscon registers on ARM Juno board,
to fully document the block and also fix dtbs_check warning:

  juno.dtb: apbregs@10000: compatible: ['syscon', 'simple-mfd'] is too short

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240518203903.119608-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>