hw/arm/sysbus-fdt: helpers for clock node generation

Some passthrough'ed devices depend on clock nodes. Those need to be
generated in the guest device tree. This patch introduces some helpers
to build a clock node from information retrieved in the host device tree.

- copy_properties_from_host copies properties from a host device tree
  node to a guest device tree node
- fdt_build_clock_node builds a guest clock node and checks the host
  fellow clock is a fixed one.

fdt_build_clock_node will become static as soon as it gets used. A
dummy pre-declaration is needed for compilation of this patch.

Signed-off-by: Eric Auger <eric.auger@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>