ARM: dts: r8a7790: Remove unnecessary clock-output-names properties

ARM: dts: r8a7790: Remove unnecessary clock-output-names properties

* Fixed rate and fixed factor clocks do not require an
  clock-output-names property.
* Since 07705583e920fef6 ("clk: shmobile: div6: Make clock-output-names
  optional") Renesas div6 clocks do not require a clock-output-names
  property.

In the above cases there is only one clock output and its name is taken
from that of the clock node.

Accordingly, remove the unnecessary clock-output-names properties and
as necessary the nodes.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>