gdbstub: Replace gdb_regs with an array

gdbstub: Replace gdb_regs with an array

An array is a more appropriate data structure than a list for gdb_regs
since it is initialized only with append operation and read-only after
initialization.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230912224107.29669-13-akihiko.odaki@daynix.com>
[AJB: fixed a checkpatch violation]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231009164104.369749-20-alex.bennee@linaro.org>