clk: sunxi: Support factor clocks with N factor starting not from 0

The PLLs on newer Allwinner SoC's, such as the A31 and A23, have a
N multiplier factor that starts from 1, not 0.

This patch adds an option to the factor clk driver's config data
structures to specify the base value of N.

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