tcl/board: Add Raspberry Pi 3 board

tcl/board: Add Raspberry Pi 3 board

OpenOCD cannot connect to BCM2837's JTAG interface on RPi 3 board until
the reset configuration mode is set as trst_only.

According to Table 6-31 GPIO Pins Alternative Function Assignment of
Broadcom's BCM2837 ARM Peripherials datasheet [1] and Raspberry Pi's
GPIO control in config.txt document [2], only Test Reset (TRST) pin (no
System Reset, SRST) is exposed.

[1] https://usermanual.wiki/Datasheet/BCM2837ARMPeripheralsBroadcom.1054296467/view
[2] https://www.raspberrypi.org/documentation/configuration/config-txt/gpio.md

Change-Id: I26ff3924039ff7943faf0a5b1ad0427c8dbb88f2
Link: http://openocd.zylin.com/#/c/6364/1
Signed-off-by: Jian-Hong Pan <chienhung.pan@gmail.com>
Reviewed-on: http://openocd.zylin.com/6366
Tested-by: jenkins
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>