hw/arm/mps2-tz: Support ROMs as well as RAMs

The AN505 and AN521 don't have any read-only memory, but the AN524
does; add a flag to ROMInfo to mark a region as ROM.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210215115138.20465-19-peter.maydell@linaro.org