doc: board: ti: k3: Add a guide to debugging with OpenOCD

Bootloader debug usually tends to be a bit dicey prior to DDR and
serial port getting active in the system. JTAG typically remains the
only practical debug option during the initial bringup.

OpenOCD is one of the most popular environment for providing debug
capability via a GDB compatible interface for developers to work with.

Debugging U-Boot and bootloaders on K3 platform does have a bit of
tribal knowledge that is better documented in our common platform
documentation.

Signed-off-by: Jason Kacines <j-kacines@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>