Some drivers are known to call the optional Map_Mem() callback without
first checking that the callback exists.  Provide a usable basic
implementation of Map_Mem() along with the other callbacks that become
mandatory if Map_Mem() is provided.

Note that in theory the PCI I/O protocol is allowed to require
multiple calls to Map(), with each call handling only a subset of the
overall mapped range.  However, the reference implementation in EDK2
assumes that a single Map() will always suffice, so we can probably
make the same simplifying assumption here.

Tested with the Intel E3522X2.EFI driver (which, incidentally, fails
to cleanly remove one of its mappings).

Originally-implemented-by: Maor Dickman <maord@mellanox.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>