acpigen: Support writing a length

acpigen: Support writing a length

It is convenient to write a length value for preceding a block of data.
Of course the length is not known or is hard to calculate a priori. So add
a way to mark the start on a stack, so the length can be updated when
known.

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>