clk: starfive: pll: Fix to use postdiv1_mask

There is a problem that the rates of PLL0 and PLL1 are set incorrectly
because the postdiv1_mask value is incorrectly entered when setting
the pll clk reg. Modify postdiv1's mask value to be put correctly.

Signed-off-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Reviewed-by: Minkyu Kang <mk7.kang@samsung.com>