dt-bindings: mfd: syscon: Document GXP register compatible

Document hpe,gxp-sysreg compatible for GXP registers.

Signed-off-by: Nick Hawkins <nick.hawkins@hpe.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20221216183532.78933-4-nick.hawkins@hpe.com