BBB tester: Introduce board DTS

We can't use the same DTS base as the standard beaglebone because
the I2C2 is used for a different purpose for the tester.

Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>
BBB-tester: Introduce cape describing the contents of the tester 'cape'

Note that the tester cape uses I2C2 for something different than an I2C
bus, so you have to load the fragment manually.

Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>