Platform/RaspberryPi: Enable NVMe boot on CM4

The CM4 has a number of carrier boards with PCIe
slots. With the PCIe changes in place its quite
possible to utilize a NVMe root device. Lets allow
people to boot from it.

Reviewed-by: Andrei Warkentin <awarkentin@vmware.com>
Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Reviewed-By: Samer El-Haj-Mahmoud <Samer.El-Haj-Mahmoud@arm.com>