emul: Add data and parent pointers to emulators

Emulators are difficult to work with, they generally require maintaining
some state in a mutable data struct. Since the emulator struct doesn't
support a data field like devices do, the pattern seems to be to add it
to the configuration. This makes following the logic of where things are
difficult.

1. Add a `struct emul *parent` structure to the espi/i2c/spi emulator
   structs to make it easier when casting up.
2. Add a `void *data` field to `struct emul` to hold the data for
   emulators.

Signed-off-by: Yuval Peress <peress@chromium.org>