Add string for _DDN method name.

The _DDN method will be used internally.
Update to GPIO region handler interface.

Changes to correct several GPIO issues:

1) The UpdateRule in a GPIO field definition is now ignored;
a read-modify-write operation is never performed for GPIO fields.
(Internally, this means that the field assembly/disassembly
code is completely bypassed for GPIO.)

2) The Address parameter passed to a GPIO region handler is
now the bit offset of the field from a previous Connection()
operator. Thus, it becomes a "Pin Number Index" into the
Connection() resource descriptor.

3) The BitWidth parameter passed to a GPIO region handler is
now the exact bit width of the GPIO field. Thus, it can be
interpreted as "number of pins".

Overall, we can now say that the region handler interface
to GPIO handlers is a raw "bit/pin" addressed interface, not
a byte-addressed interface like the SystemMemory handler interface.

For Linux, this patch will need to be applied simultaneously
with the Linux GPIO handler patch that supports it.