clk: shmobile: Add R-Car Gen2 RCAN clock support

Add the RCAN clock support to the R-Car generation 2 CPG driver.  This clock
gets derived from  the USB_EXTAL clock, dividing  it by 6.  The layout of the
RCANCKCR register is similar to those of the clocks supported by the 'clk-div6'
driver but has no divider field, and so can't be supported by that driver...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>