There are two methods under VPC2004 which is used to access VDAT/VCMD of EC
register. Add helpers for read and write these two registers.

And add read_method_int for reading the return value from ACPI methods which
requires no parameter.
Signed-off-by: Ike Panhc <ike.pan@canonical.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>