usb: gadget: udc: document our sysfs ABI

I noticed that this has been missing for quite
some time so I decided it was about time to
document it.
Signed-off-by: NFelipe Balbi <balbi@ti.com>

Documentation/ABI/stable/sysfs-class-udc

+What:		/sys/class/udc/<udc>/a_alt_hnp_support
+Date:		June 2011
+KernelVersion:	3.1
+Contact:	Felipe Balbi <balbi@kernel.org>
+Description:
+		Indicates if an OTG A-Host supports HNP at an alternate port.
+Users:
+
+What:		/sys/class/udc/<udc>/a_hnp_support
+Date:		June 2011
+KernelVersion:	3.1
+Contact:	Felipe Balbi <balbi@kernel.org>
+Description:
+		Indicates if an OTG A-Host supports HNP at this port.
+Users:
+
+What:		/sys/class/udc/<udc>/b_hnp_enable
+Date:		June 2011
+KernelVersion:	3.1
+Contact:	Felipe Balbi <balbi@kernel.org>
+Description:
+		Indicates if an OTG A-Host enabled HNP support.
+Users:
+
+What:		/sys/class/udc/<udc>/current_speed
+Date:		June 2011
+KernelVersion:	3.1
+Contact:	Felipe Balbi <balbi@kernel.org>
+Description:
+		Indicates the current negotiated speed at this port.
+Users:
+
+What:		/sys/class/udc/<udc>/is_a_peripheral
+Date:		June 2011
+KernelVersion:	3.1
+Contact:	Felipe Balbi <balbi@kernel.org>
+Description:
+		Indicates that this port is the default Host on an OTG session
+		but HNP was used to switch roles.
+Users:
+
+What:		/sys/class/udc/<udc>/is_otg
+Date:		June 2011
+KernelVersion:	3.1
+Contact:	Felipe Balbi <balbi@kernel.org>
+Description:
+		Indicates that this port support OTG.
+Users:
+
+What:		/sys/class/udc/<udc>/maximum_speed
+Date:		June 2011
+KernelVersion:	3.1
+Contact:	Felipe Balbi <balbi@kernel.org>
+Description:
+		Indicates the maximum USB speed supported by this port.
+Users:
+
+What:		/sys/class/udc/<udc>/maximum_speed
+Date:		June 2011
+KernelVersion:	3.1
+Contact:	Felipe Balbi <balbi@kernel.org>
+Description:
+		Indicates the maximum USB speed supported by this port.
+Users:
+
+What:		/sys/class/udc/<udc>/soft_connect
+Date:		June 2011
+KernelVersion:	3.1
+Contact:	Felipe Balbi <balbi@kernel.org>
+Description:
+		Allows users to disconnect data pullup resistors thus causing a
+		logical disconnection from the USB Host.
+Users:
+
+What:		/sys/class/udc/<udc>/srp
+Date:		June 2011
+KernelVersion:	3.1
+Contact:	Felipe Balbi <balbi@kernel.org>
+Description:
+		Allows users to manually start Session Request Protocol.
+Users:
+
+What:		/sys/class/udc/<udc>/state
+Date:		June 2011
+KernelVersion:	3.1
+Contact:	Felipe Balbi <balbi@kernel.org>
+Description:
+		Indicates current state of the USB Device Controller. Valid
+		states are: 'not-attached', 'attached', 'powered',
+		'reconnecting', 'unauthenticated', 'default', 'addressed',
+		'configured', and 'suspended'; however not all USB Device
+		Controllers support reporting all states.
+Users: