target/arm: generate a custom MIDR for -cpu max

target/arm: generate a custom MIDR for -cpu max

While most features are now detected by probing the ID_* registers
kernels can (and do) use MIDR_EL1 for working out of they have to
apply errata. This can trip up warnings in the kernel as it tries to
work out if it should apply workarounds to features that don't
actually exist in the reported CPU type.

Avoid this problem by synthesising our own MIDR value.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190726113950.7499-1-alex.bennee@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>