clk: sunxi-ng: use 1 as fallback for minimum multiplier

A zero multiplier does not make sense for clocks.

Use 1 as the minimum when a multiplier minimum isn't specified.

Fixes: 2beaa601 ("clk: sunxi-ng: Implement minimum for multipliers")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>