It's useful for bootloader to do I/O operations.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Huacai Chen <chenhuacai@kernel.org>
Message-Id: <20201215064507.30148-3-jiaxun.yang@flygoat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>