acpi: Add an __ACPI__ preprocessor symbol

The ASL compiler cannot handle C structures and the like so needs some
sort of header guard around these.

We already have an __ASSEMBLY__ #define but it seems best to create a new
one for ACPI since the rules may be different.

Add the check to a few files that ACPI always includes.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>