arm64: dts: fvp: Align virtio device node names with dtschema

Align the virtio mmio device tree node names with the schema to avoid any
schema warnings.

Link: https://lore.kernel.org/r/20220425135524.1077986-1-sudeep.holla@arm.com


Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>