emul: mspi: Add the mspi controller emulator

Add bus emulator support for MSPI and the MSPI controller emulator.
The mspi_emul.c not only serves as an emulator but also provides an
example implementation of the MSPI API. It does not actually do anything
other than validating parameters and forwarding transceive request back
to the device driver emulators.

Signed-off-by: Swift Tian <swift.tian@ambiq.com>