intel-iommu: support 64K domains with 16-bit id

Change the definition of the number of domains supported. Now the Capability
Register reports that 16-bit domain id is supported.
VTD_DOMAIN_ID_SHIFT should be used to check context-entry which has not been
done yet.

Signed-off-by: Le Tan <tamlokveer@gmail.com>
intel-iommu: clear up the dfinitions of MGAW and SAGAW in Capability Register

Add some macros for the definitions of MGAW and SAGAW in Capability Register
instead of instant number. These two fields may be used in translation.

Signed-off-by: Le Tan <tamlokveer@gmail.com>