spi: dt-bindings: add documentation for hpe,gxp-spifi

Create documentation for the hpe,gxp-spifi binding to support access to
the SPI parts

Signed-off-by: Nick Hawkins <nick.hawkins@hpe.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220728161459.7738-3-nick.hawkins@hpe.com
Signed-off-by: Mark Brown <broonie@kernel.org>