dt-bindings: riscv: microchip: document the sev kit

Update devicetree bindings document with PolarFire SoC Video Kit, known
by its "sev-kit" product code.

Link: https://onlinedocs.microchip.com/pr/GUID-404D3738-DC76-46BA-8683-6A77E837C2DD-en-US-1/index.html?GUID-065AEBEE-7B2C-4895-8579-B1D73D797F06


Signed-off-by: Shravan Chippa <shravan.chippa@microchip.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>