The 'peer-hub' is a valid property for the hub.

Document it to fix the following dt-schema warning:

imx8mp-debix-som-a-bmb-08.dtb: hub@1: 'peer-hub' does not match any of the regexes: 'pinctrl-[0-9]+'
	from schema $id: http://devicetree.org/schemas/usb/genesys,gl850g.yaml#Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20231207124217.2530457-1-festevam@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>