dts: pwm: npcx: add override mechanism for pwm input clock assignment.

This CL adds a override mechanism for pwm module's input clock source
assignment. If the 'clock-bus' property exists, the NPCX_DT_PROP_ENUM_OR
macro function will return an enum upper token value. Otherwise, it
expands to default value in 'clocks' property.

For example, if the users want to select LFCLK as pwm0's input clock,
ths node can be overridden by adding 'clock-bus' property with an enum
string, "NPCX_CLOCK_BUS_LFCLK".

&pwm0 {
	status = "okay";
	clock-bus = "NPCX_CLOCK_BUS_LFCLK";
};

Signed-off-by: Keith Short <keithshort@google.com>
Signed-off-by: Mulin Chao <mlchao@nuvoton.com>