ARM: dts: r8a7794: add IIC clocks

Add IIC clocks to r8a7794 device tree.

Based on similar work for the r8a7790 by Wolfram Sang.

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