Make pci_host_generic and thunderx_pci common

• provided OFW interface for pci_host_generic (for handling devices which are present in DTS under the PCI node)
• removed support for internal PCI from arm64/cavium
• cleaned up and made most of the code common

Obtained from: Semihalf
Sponsored by: Cavium
Approved by: cognet (mentor)
Reviewed by: zbb
Differential revision: https://reviews.freebsd.org/D5261