RFC: arm: pci: Add PCI cam support to PCI-E ecam driver

RFC: arm: pci: Add PCI cam support to PCI-E ecam driver

When booting U-Boot in crosvm, the virtual machine emulates a PCI cam
device, not the PCI-E 'ecam' device normally seen on e.g. QEMU. This
PCI device can be supported with only trivial changes to the ecam
driver.

Instead of adding a completely new driver which is identical besides the
initialization step, add support for the PCI version to the existing
driver.

Signed-off-by: Alistair Delva <adelva@google.com>
Cc: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
Cc: Ram Muthiah <rammuthiah@google.com>