include/exec/memattrs: Add two bits of space to MemTxAttrs

include/exec/memattrs: Add two bits of space to MemTxAttrs

We will need 2 bits to represent ARMSecurityState.

Do not attempt to replace or widen secure, even though it
logically overlaps the new field -- there are uses within
e.g. hw/block/pflash_cfi01.c, which don't know anything
specific about ARM.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20230620124418.805717-7-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>