emul: Simplify emulator bus initialization and API

Allow emulator creators to write an init function that can be used
across multiple busses so as to reduce the boilerplate and cognitive
load in creating an emulator.

Part of this change includes allowing access to the emul struct from a
field in a {bus}_struct api (e.g. i2c_struct), which removes the need for
sporadic usages of CONTAINER_OF to access the emul struct.

Overall, this change simplifies and reduces the amount of boilerplate
code to get a device emulator up and running, thus reducing excise work
to writing tests.

TEST=twister on accel,espi, and eeprom drivers tests

Signed-off-by: Aaron Massey <aaronmassey@google.com>
11 files changed