jtag/drivers/bcm2835gpio: bcm2835gpio_init has been updated

jtag/drivers/bcm2835gpio: bcm2835gpio_init has been updated

For jtag/drivers/bcm2835gpio dev_mem_fd has been updated within
bcm2835gpio_init with the add on of gpio to mem. This permits the
access to memory of GPIO without the need for root access.
For failed attempt, a fallback to original memory follows.
It should be noted that any printed error is relative to original
memory ("dev/mem").
Tested on EFM32GG12B390F board

Change-Id: I4540bdf62fb3b91a51221e277881adfae138dcc5
Signed-off-by: tscn92 <tscn@kamstrup.com>
Reviewed-on: http://openocd.zylin.com/5568
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>