acpi: Add support for DSDT generation

Some devices need to inject extra code into the Differentiated System
Descriptor Table (DSDT). Add a method to handle this.

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: correct one typo in inject_dsdt() comments]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>