clk: hi6220: change watchdog clock source

The old code uses tcxo (19.2MHz) as watchdog clock but actually the
watchdog uses 32K clock, as result the watchdog timeout cannot be set
correctly and delay long time to reset SoC.

So this patch is to use 'ref32k' as clock source for watchdog.

Fixes: 72ea4861 ("clk: hi6220: Clock driver support for Hisilicon hi6220 SoC")
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>