ARM: dts: r8a7793: Remove unnecessary clock-output-names properties

ARM: dts: r8a7793: 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 update the node names.

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