doc/openocd.texi: Add documentation for bcm2835 interface

doc/openocd.texi: Add documentation for bcm2835 interface

This adds documentation for the bcm2835 interface configuration
parameters to the user manual. Documentation format is based on
the FTDI interface section, and was taken from the descriptions
in the driver source code.

Change-Id: I77b09b8bd44d8e8fe9cc5fb9de3c3a30550d943c
Signed-off-by: Matthew Mets <matt@blinkinlabs.com>
Reviewed-on: http://openocd.zylin.com/6376
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>