drivers/bcm2835: Add support for SWDIO direction control pin

drivers/bcm2835: Add support for SWDIO direction control pin

Adds a new, optional configuration "bcm2835gpio_swdio_dir_num" to
the BCM2835 driver, to control the direction of an external buffer
driver IC in SWD mode. For example, this is needed to use a level-
shifting buffer, such as the SN74LVC2T45 used on the JTAG Hat

Change-Id: If5c146f310ecf8ceae85443b3670936467d2786d
Signed-off-by: Matthew Mets <matt@blinkinlabs.com>
Reviewed-on: http://openocd.zylin.com/6371
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins