hw/char/pl011: Restrict MemoryRegionOps implementation access sizes

The pl011_read() and pl011_write() handlers shift the offset
argument by 2, so are implemented on a 32-bit boundary.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230710175102.32429-2-philmd@linaro.org>