BCM270X_DT: Improve i2c-sensor and i2c-rtc overlay

Use the "__dormant__" feature to permit multiple instances of each
overlay, which is more useful now that changing the "reg" property
also changes the node address. Although the overlay grows slightly,
when applied only the requested node is included.

Usage does not change, except that the "lm75addr" parameter of the
i2c-sensor overlay has been deprecated in favour of the generic
"addr" parameter.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>