target/arm: hide all versions of DBGD[RS]AR from gdbstub

This avoids two duplicates being presented to gdbstub. As the
registers are RAZ anyway it is unlikely their value would be of use to
someone using gdbstub anyway.

Acked-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231106185112.2755262-5-alex.bennee@linaro.org>