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

Add the ADSP clock support to the R-Car generation 2 CPG driver.  This clock
gets derived from  PLL1.  The layout of the ADSPCKCR register is  similar to
those of the clocks supported by the 'clk-div6' driver but the divider encoding
is non-linear, so can't be supported by that driver...

Based on the original patch by Konstantin Kozhevnikov
<konstantin.kozhevnikov@cogentembedded.com>.

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