ppc/pnv: I2C controller is not user creatable

ppc/pnv: I2C controller is not user creatable

The I2C controller is a subunit of the processor. Make it so and avoid
QEMU crashes.

  $ build/qemu-system-ppc64 -S -machine powernv9 -device pnv-i2c
  qemu-system-ppc64: ../hw/ppc/pnv_i2c.c:521: pnv_i2c_realize: Assertion `i2c->chip' failed.
  Aborted (core dumped)

Fixes: 263b81ee15af ("ppc/pnv: Add an I2C controller model")
Cc: Glenn Miles <milesg@linux.vnet.ibm.com>
Reported-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Glenn Miles <milesg@linux.vnet.ibm.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>