linuxkpi: Add more pci functions needed by DRM

-pci_get_class : This function search for a matching pci device based on

the class/subclass and returns a newly created pci_dev.

• pci_{save,restore}_state : This is analogous to ours with the same name
• pci_is_root_bus : Return true if this is the root bus
• pci_get_domain_bus_and_slot : This function search for a matching pci device based on domain, bus and slot/function concat into a single unsigned int (devfn) and returns a newly created pci_dev
• pci_bus_{read,write}_config* : Read/Write to the config space.

While here add some helper function to alloc and fill the pci_dev struct.

Reviewed by: hselasky, bz (older version)
Differential Revision: https://reviews.freebsd.org/D27550