goldfish_pipe: Squash warnings on 32-bit systems

goldfish_pipe: Squash warnings on 32-bit systems

The goldfish_pipe driver casts a pointer to a (u64), but this will
generate a warning on 32-bit systems, because we are casting a pointer
to a type with a different size.  Instead, cast the pointer to an
unsigned long, which will work on both 32-bit and 64-bit systems, and
then cast that to a 32-bit value which the backend expects.

Also wrap this in a pipe_id() function to make it clear that we are not
expecting the backend to use the pointer for anything but that we are
merely using it as an identifier.  Perhaps it would be possible to use a
simple running allocation scheme of IDs instead.

Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>