pciconf: Use VM_MEMATTR_DEVICE on supported architectures

Some architectures - armv7, armv8 and riscv use VM_MEMATTR_DEVICE
when mapping device registers in kernel. Do the same in pciconf.
On armada8k SoC all reads from BARs mapped with hitherto attribute
(VM_MEMATTR_UNCACHEABLE) return 0xff's.

Submitted by: Kornel Duleba <mindal@semihalf.com>
Reviewed by: kib
Obtained from: Semihalf
Sponsored by: Marvell
Differential revision: https://reviews.freebsd.org/D29603