clk: sunxi: clean the magic number of mux parents

This was pointed out during the review of the factor patches. Let's
indicate what does that magic 5 mean.

Signed-off-by: Emilio López <emilio@elopez.com.ar>
Acked-by: Mike Turquette <mturquette@linaro.org>

diff --cc drivers/clk/sunxi/clk-sunxi.c
Simple merge