ite: drivers/adc: create pinmux phandle to the ADC driver node

Create the pinmux phandle to the ADC driver node in the
devicetree. When the pinmux_pin_set function in
adc_it8xxx2_channel_setup can refer to the setting of
this phandle. It is more flexible to use.

Signed-off-by: Tim Lin <tim2.lin@ite.corp-partner.google.com>