arm: Don't let no-MPU PMSA cores write to SCTLR.M

arm: Don't let no-MPU PMSA cores write to SCTLR.M

If the CPU is a PMSA config with no MPU implemented, then the
SCTLR.M bit should be RAZ/WI, so that the guest can never
turn on the non-existent MPU.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 1493122030-32191-7-git-send-email-peter.maydell@linaro.org