hw/arm/boot: Fix MTE for EL3 direct kernel boot

hw/arm/boot: Fix MTE for EL3 direct kernel boot

When booting an EL3 cpu with -kernel, we set up EL3 and then
drop down to EL2.  We need to enable access to v8.5-MemTag
tag allocation at EL3 before doing so.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200724163853.504655-3-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>