ARM: shmobile: r8a7791: 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 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.

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