hw/arm/aspeed: allow missing spi_model

hw/arm/aspeed: allow missing spi_model

Generally all BMCs will use the fmc_model to hold their own flash
and most will have a spi_model to hold the managed system's flash,
but not all systems do.  Add a simple NULL check to allow a system
to set the spi_model as NULL to indicate it should not be instantiated.

Signed-off-by: Patrick Williams <[email protected]>
Message-Id: <20220305000656.1944589[email protected]>
Signed-off-by: Cédric Le Goater <[email protected]>