clk: nuvoton: fix bug for calculate pll clock

Fix bug for npcm7xx bmc calculate pll clock.
PLLCON1 need to divide by 2.

Signed-off-by: Jim Liu <JJLIU0@nuvoton.com>
Acked-by: Sean Anderson <seanga2@gmail.com>
Link: https://lore.kernel.org/r/20221121091528.1351-1-JJLIU0@nuvoton.com