ipxe/ipxe

Some BIOSes in AWS EC2 (observed with a c6i.metal instance in
eu-west-2) will fail to assign an MMIO address to the ENA device,
which causes ioremap() to fail.

Experiments show that the ENA device is the only device behind its
bridge, even when multiple ENA devices are present, and that the BIOS
does assign a memory window to the bridge.

We may therefore choose to assign the device an MMIO address at the
start of the bridge's memory window.

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