clk: loongson2: Allow zero divisors for dividers

LS2K0300 and LS2K0500 ship divider clocks which allows zero divisors,
in which case the divider acts the same as one is specified.

Let's pass CLK_DIVIDER_ALLOW_ZERO when registering divider clocks to
prepare for future introduction of these clocks.

Signed-off-by: Yao Zi <ziyao@disroot.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>