dts: arm: npcx: fix clock source of spi_fiu node

The clock bus of the specific SPI module (FIU) is under FIUCLK. The
previous commit misconfigured it to APB3_CLK. Although it won't
cause any issue as the driver doesn't use it. It should be fixed to
prevent confusion when reading the datasheet.

Signed-off-by: Jun Lin <CHLin56@nuvoton.com>