Documentation: devicetree: add Broadcom GISB arbiter bindings

This patch adds the Broadcom GISB arbiter Device Tree binding that is
used on all Broadcom Set-top-box System-on-a-chip.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>