pci: Map bars with offset and length

Evolve dm_pci_map_bar() to include an offset and length parameter. These
allow a portion of the memory to be mapped and range checks to be
applied.

Passing both the offset and length as zero results in the previous
behaviour and this is used to migrate the previous callers.

Signed-off-by: Andrew Scull <ascull@google.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>