target/xtensa: virtualize XDM registers

target/xtensa: virtualize XDM registers

Use indirect enum IDs to access XDM registers in preparation for supporting both NAR (JTAG) and APB (DAP).  No new clang static analysis warnings.

Signed-off-by: Ian Thompson <ianst@cadence.com>
Change-Id: I0b742fe4661ff3cf609454b8650493d141a1e1ff
Reviewed-on: https://review.openocd.org/c/openocd/+/7143
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>