target/adi_v5_jtag: Add support for 8-bit IR JTAG-DP

target/adi_v5_jtag: Add support for 8-bit IR JTAG-DP

As per Arm Debug Interface Architecture Specification (ADIv5.0 to
ADIv5.2), B3.3.1, the JTAG-DP as an IR length of 4 or 8 bits
depending on the ARM implementation. The current code
only support 4-bit and this patch extends the support to 8-bit IR.
Not tested back yet on a 4-bit target.

Change-Id: Ie4f875dc336caf014c6cfced57574b54d0970623
Signed-off-by: Antoine C. <acalando@free.fr>
Reviewed-on: https://review.openocd.org/c/openocd/+/6285
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>