[ena] Map the on-device memory, if present

[ena] Map the on-device memory, if present

Newer generations of the ENA hardware require the use of low latency
transmit queues, where the submission queues and the initial portion
of the transmitted packet are written to on-device memory via BAR2
instead of being read from host memory.

Prepare for this by mapping the on-device memory BAR.  As with the
register BAR, we may need to steal a base address from the upstream
PCI bridge since the BIOS on some instance types (observed with an
m8i.metal-48xl instance in eu-south-2) will fail to assign an address
to the device.

Signed-off-by: Michael Brown <mcb30@ipxe.org>