ARM: dts: r8a7792: add CAN clocks

The R-Car CAN controllers can derive  the CAN  bus  clock not only from
their peripheral  clock input (clkp1) but also from the other internal
clock (clkp2) and the external clock fed on the CAN_CLK pin.  Describe
those  clocks in  the R8A7792 device tree.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>