clk: nuvoton: add read only feature for clk driver

Add a flag to set ahb/apb/fiu/spi clock divider as read-only
The spi clock setting is related to booting flash, it is setup by early
bootloader.
It just protects the clock source and can't modify it in uboot.

Signed-off-by: Jim Liu <JJLIU0@nuvoton.com>
Reviewed-by: Sean Anderson <seanga2@gmail.com>
Link: https://lore.kernel.org/r/20231114090004.3746024-1-JJLIU0@nuvoton.com