hw/ppc: SPI controller wiring to P10 chip

hw/ppc: SPI controller wiring to P10 chip

In this commit, create SPI controller on p10 chip and connect cs irq.

The QOM tree of pnv-spi and seeprom are.
/machine (powernv10-machine)
  /chip[0] (power10_v2.0-pnv-chip)
    /pib_spic[2] (pnv-spi)
      /pnv-spi-bus.2 (SSI)
      /xscom-spi[0] (memory-region)

/machine (powernv10-machine)
  /peripheral-anon (container)
    /device[0] (25csm04)
      /WP#[0] (irq)
      /ssi-gpio-cs[0] (irq)

(qemu) qom-get /machine/peripheral-anon /device[76] "parent_bus"
"/machine/chip[0]/pib_spic[2]/pnv-spi-bus.2"

Signed-off-by: Chalapathi V <chalapathi.v@linux.ibm.com>
Reviewed-by: Glenn Miles <milesg@linux.ibm.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>