When memory region is ram, the lower TARGET_PAGE_BITS is not the
physical section number. Instead, its value is always 0.

Add comment and assert to make it clear.

Signed-off-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Message-Id: <20230901060118.379-1-zhiwei_liu@linux.alibaba.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>