arm64: dts: fvp: Move fixed clocks out of bus node

The devicetree compiler complains when DT nodes without a reg property
live inside a (simple) bus node:
Warning (simple_bus_reg): Node /bus@8000000/v2m_refclk32khz
                          missing or empty reg/ranges property

Move the fixed clocks to the root node, since they do not depend on any
busses.

Link: https://lore.kernel.org/r/20200513103016.130417-6-andre.przywara@arm.com


Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>