x86: acpi: Store the ACPI context in global_data

At present we create the ACPI context but then drop it after generation of
tables is complete. This is annoying because we have to then search for
tables later.

To fix this, allocate the context and store it in global_data.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>