board: ti: cc1352p7_lp: Add complementary instructions about OpenOCD

Currently, OpenOCD doesn't support the cc1352p7.
There are two open PR adding its support to Zephyr's OpenOCD[1]
and upstream OpenOCD[2].
Until this gets merged, we need to use a downstream version of OpenOCD.
This updates the documentation to explain where to find it and how to
use it.

[1] https://github.com/zephyrproject-rtos/openocd/pull/65
[2] https://review.openocd.org/q/owner:abailon%2540baylibre.com



Signed-off-by: Alexandre Bailon <abailon@baylibre.com>