arm_adi_v5: move in a separate function devtype decode/display

arm_adi_v5: move in a separate function devtype decode/display

For readability, move in a separate function the decoding and the
display of devtype register.
The function will be reused with ADIv6.

Split from change https://review.openocd.org/6077/

Change-Id: I7a26a2c9759d5db5f9acfae5c169b90b3deb2f18
Signed-off-by: Kevin Burke <kevinb@os.amperecomputing.com>
Signed-off-by: Daniel Goehring <dgoehrin@os.amperecomputing.com>
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6448
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>