ls1046ardb: Add support for JTAG boot

ls1046ardb: Add support for JTAG boot

This adds support for booting entirely from JTAG while using a
hard-coded RCW. With these steps, it is not necessary to program a
"good" RCW using CodeWarrior. The method here can be performed with any
JTAG adapter supported by OpenOCD, including the on-board CMSIS-DAP
(albeit very slowly).

These steps require LS1046A support in OpenOCD, which was added in [1].

[1] https://sourceforge.net/p/openocd/code/ci/5b70c1f679755677c925b4e6dd2c3d8be4715717/

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
[trini: Add reference to doc/board/nxp/ls1046ardb.rst]