aspeed/smc: support different memory region ops for SMC flash region

aspeed/smc: support different memory region ops for SMC flash region

It set "aspeed_smc_flash_ops" struct which containing
read and write callbacks to be used when I/O is performed
on the SMC flash region. And it set the valid max_access_size 4
by default for all ASPEED SMC models.

However, the valid max_access_size 4 only support 32 bits CPUs.
To support all ASPEED SMC model, introduce a new
"const MemoryRegionOps *" attribute in AspeedSMCClass and
use it in aspeed_smc_flash_realize function.

Signed-off-by: Troy Lee <troy_lee@aspeedtech.com>
Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>