acpi: Move the table-finding functions into the libary

This is useful for other features. Move the function into library code
so it can be used outside just the 'acpi' command.

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