dts: gen_defines: Remove lots of code duplication for phandle/val lists

Add a write_phandle_val_list() function for handling GPIOs, PWMs, and IO
channels. The logic is the same in all cases.

This also indirectly makes pwm-names and io-channel-names work the same
as gpio-names. Previously, they were ignored.

Also add a long explanation with example output.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>