ARM: dts: r8a7793: add CAN clocks to device tree

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 external clock fed on CAN_CLK pin. Describe those clocks in
the device tree along with  the USB_EXTAL clock from which clkp2 is
derived.

Based on work by Sergei Shtylyov for the r8a7791 SoC.

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