Documentation: sysfs: add Broadcom GISB arbiter sysfs properties

This patch documents the sysfs properties exposed by the Broadcom GISB
bus arbiter hardware.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Documentation/ABI/testing/sysfs-platform-brcmstb-gisb-arb

+What:		/sys/devices/../../gisb_arb_timeout
+Date:		May 2014
+KernelVersion:	3.17
+Contact:	Florian Fainelli <f.fainelli@gmail.com>
+Description:
+		Returns the currently configured raw timeout value of the
+		Broadcom Set Top Box internal GISB bus arbiter. Minimum value
+		is 1, and maximum value is 0xffffffff.