ARM: dts: s5pv210: move fixed clocks under root node

The fixed clocks are kept under dedicated 'external-clocks' node, thus a
fake 'reg' was added.  This is not correct with dtschema as fixed-clock
binding does not have a 'reg' property.  Moving fixed clocks out of
'soc' to root node fixes multiple dtbs_check warnings:

  external-clocks: $nodename:0: 'external-clocks' does not match '^([a-z][a-z0-9\\-]+-bus|bus|soc|axi|ahb|apb)(@[0-9a-f]+)?$'
  external-clocks: #size-cells:0:0: 0 is not one of [1, 2]
  external-clocks: oscillator@0:reg:0: [0] is too short
  external-clocks: oscillator@1:reg:0: [1] is too short
  external-clocks: 'ranges' is a required property
  oscillator@0: 'reg' does not match any of the regexes: 'pinctrl-[0-9]+'

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Tested-by: Jonathan Bakker <xc-racer2@live.ca>
Link: https://lore.kernel.org/r/20200907161141.31034-7-krzk@kernel.org