clk: renesas: div6: Document fields used for parent selection

Add the missing documentation for the fields in struct div6_clock
related to parent selection for DIV6 clocks with selectable parents, as
found in R/SH-Mobile SoCs.

Fixes: c6d67fb0 ("clk: shmobile: div6: support selectable-input clocks")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>