Currently our PMSAv7 and ARMv7M MPU implementation cannot handle
MPU region sizes smaller than our TARGET_PAGE_SIZE. However we
report that in a slightly confusing way:

 DRSR[3]: No support for MPU (sub)region alignment of 9 bits. Minimum is 10

The problem is not the alignment of the region, but its size;
tweak the error message to say so:
 DRSR[3]: No support for MPU (sub)region size of 512 bytes. Minimum is 1024.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180405172554.27401-1-peter.maydell@linaro.org