clk: renesas: cpg-mssr: add generic support for read-only DIV6 clocks

Gen3 has two clocks (OSC and R) which look like a DIV6 clock but their
divider value is read-only and depends on MD pins at bootup. Add support
for such clocks by reading the value and adding a fixed clock.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>