clk: sunxi: factors: Make struct clk_factors_config table const

struct clk_factors_config contains shifts/widths for the factors of
the factors clk. This is used to read out the factors from the register
value. In no case is it written to, so make it const.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>