Add the IMR[0-3] clocks to the R8A7795 CPG/MSSR driver.

Based on the original (and large) patch by Konstantin Kozhevnikov
<Konstantin.Kozhevnikov@cogentembedded.com>.
Signed-off-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>