riscv: enable generic PCI resource mapping

Enable the PCI resource mapping on RISC-V using the generic framework.
This allows userspace applications to mmap PCI resources using
/sys/devices/pci*/*/resource* interface.
The mmap has been tested with Intel x520-DA2 NIC card on a HiFive
Unmatched board (SiFive FU740 SoC).

Signed-off-by: Stanislaw Kardach <kda@semihalf.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>