target/arm: Allow cpu to configure GM blocksize

target/arm: Allow cpu to configure GM blocksize

Previously we hard-coded the blocksize with GMID_EL1_BS.
But the value we choose for -cpu max does not match the
value that cortex-a710 uses.

Mirror the way we handle dcz_blocksize.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20230811214031.171020-3-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>