hw/ssi: Add SPI model

hw/ssi: Add SPI model

SPI controller device model supports a connection to a single SPI responder.
This provide access to SPI seeproms, TPM, flash device and an ADC controller.

All SPI function control is mapped into the SPI register space to enable full
control by firmware. In this commit SPI configuration component is modelled
which contains all SPI configuration and status registers as well as the hold
registers for data to be sent or having been received.

An existing QEMU SSI framework is used and SSI_BUS is created.

Signed-off-by: Chalapathi V <chalapathi.v@linux.ibm.com>
Reviewed-by: Caleb Schlossin <calebs@linux.vnet.ibm.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Glenn Miles <milesg@linux.ibm.com>
[np: Fix FDT macro compile for qtest]
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>