Remove a possible source of error by removing REGINFO_SENTINEL
and using ARRAY_SIZE (convinently hidden inside a macro) to
find the end of the set of regs being registered or modified.

The space saved by not having the extra array element reduces
the executable's .data.rel.ro section by about 9k.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220501055028.646596-4-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>