acpi: Export functions to write sized values

At present only acpigen_write_integer() is exported for use by other code.
But in some cases it is useful to call the specific function depending on
the size of the value.

Export these functions and add a test.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: Fix the "new blank line at EOF" warning]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>