acpi: Create a new Kconfig for ACPI

We have several Kconfig options for ACPI, but all relate to specific
functions, such as generating tables and AML code.

Add a new option which controls including basic ACPI library code,
including the lib/acpi directory. This will allow us to add functions
which are available even if table generation is not supported.

Adjust the command to avoid a build error when ACPIGEN is not enabled.

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