target/arm: Adjust and validate mtedesc sizem1

target/arm: Adjust and validate mtedesc sizem1

When we added SVE_MTEDESC_SHIFT, we effectively limited the
maximum size of MTEDESC.  Adjust SIZEM1 to consume the remaining
bits (32 - 10 - 5 - 12 == 5).  Assert that the data to be stored
fits within the field (expecting 8 * 4 - 1 == 31, exact fit).

Cc: qemu-stable@nongnu.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Gustavo Romero <gustavo.romero@linaro.org>
Message-id: 20240207025210.8837-4-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
(cherry picked from commit b12a7671b6099a26ce5d5ab09701f151e21c112c)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>