hw/ssi: Extend SPI model

hw/ssi: Extend SPI model

In this commit SPI shift engine and sequencer logic is implemented.
Shift engine performs serialization and de-serialization according to the
control by the sequencer and according to the setup defined in the
configuration registers. Sequencer implements the main control logic and
FSM to handle data transmit and data receive control of the shift engine.

Signed-off-by: Chalapathi V <chalapathi.v@linux.ibm.com>
Reviewed-by: Caleb Schlossin <calebs@linux.vnet.ibm.com>
Reviewed-by: Glenn Miles <milesg@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>