Add a PCI driver that follows the Arm DEN0115 spec

Add a n attachment to the pci_host_generic driver for the Arm DEN0115
PCI Configuration Space Access Firmware Interface [1]. This can be used
when PCI controllers need to implement quirks in the PCI root bus.
To handle this the firmware implements a SMCCC interface the driver can
use to read and write the configuration register.

This has been tested on a Raspberry Pi 4 booting with EDK2.

[1] https://developer.arm.com/documentation/den0115/latest

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D39228