arm64: dts: fvp: Add virtio-rng support

The virtio-rng is available from FVP_Base_RevC-2xAEMvA version 11.17,
so add the devicetree node to support it. It is disabled by default to
avoid any issues with models that doesn't support it.

Link: https://lore.kernel.org/r/ac3be672c636091ee1e079cadce776b1fb7e0b2e.1650543392.git.diego.sueiro@arm.com


Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>