tcl/board: Add Raspberry Pi 4 model B board

tcl/board: Add Raspberry Pi 4 model B board

OpenOCD cannot connect to BCM2711's JTAG interface on RPi 4B board until
the reset configuration mode is set as trst_only.

According to Table 94. GPIO Pins Alternative Function Assignment of
Broadcom's BCM2711 ARM Peripherals 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://datasheets.raspberrypi.org/bcm2711/bcm2711-peripherals.pdf
[2] https://www.raspberrypi.org/documentation/configuration/config-txt/gpio.md

Change-Id: I806f0be9700fa0f0944b42c8a651a5731adc762b
Link: http://openocd.zylin.com/#/c/6364/1
Signed-off-by: Jian-Hong Pan <chienhung.pan@gmail.com>
Reviewed-on: http://openocd.zylin.com/6367
Tested-by: jenkins
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>