BCM270X_DT: Allow multiple instances of w1-gpio overlays

Upcoming firmware will modify the address portion of node names when
their "reg" property is written by a dtparam. Modify the w1-gpio
overlays to write the gpiopin parameter value to "reg" properties, so
that multiple instances can be loaded simultaneously.

Note: The value of the "address" is unimportant - the w1 subsystem
assigns instance numbers to buses sequentially from 1, and it is
not necessary to know which bus a device is on in order to find it.