debug: gdbstub: Move to DTS for uart device

Move from using Kconfig GDBSTUB_SERIAL_BACKEND_NAME to a devicetree
chosen property ("zephyr,gdbstub-uart"). This is similar to a number
of other functions like "zephyr,shell-uart" or "zephyr,bt-uart".

Signed-off-by: Benjamin Björnsson <benjamin.bjornsson@gmail.com>