ACPI: EC: Use fast path in acpi_ec_add() for DSDT boot EC

If the boot EC comes from the DSDT, its ACPI handle is equal to the
handle of a device object with the PNP0C09 device ID.  If that
device object is passed to acpi_ec_add(), it is not necessary to
allocate a new EC structure for it and parse it, because that has
been done already, so change the function to use the fast path in
that case.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>