tcl/target: add BCM2837 configuration file

tcl/target: add BCM2837 configuration file

This is the Broadcom chip used in the Raspberry Pi 3,
and in later models of the Raspberry Pi 2.

Partial information is available in raspberry pi website:
https://www.raspberrypi.org/documentation/hardware/raspberrypi/bcm2837
https://www.raspberrypi.org/documentation/hardware/raspberrypi/bcm2837b0

Change-Id: I1188a7866304c59f670a543809aca3927174786e
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/6069
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>