libpci: Move physical memory mapping mmap() code from ecam/mmio-ports to physmem...

libpci: Move physical memory mapping mmap() code from ecam/mmio-ports to physmem-posix.c file

This deduplicates physical memory mapping mmap() code found in ecam and
mmio-ports backends into common functions with new physmem API.

This new physmem API allows to implement also non-mmap() variants of
physical memory mapping.