rtc: max77686: convert to devm_i2c_new_dummy_device()

I was about to simplify the call to i2c_unregister_device() when I
realized that converting to devm_i2c_new_dummy_device() will simplify
the driver a lot. So I took this approach.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20190830133124.21633-2-wsa+renesas@sang-engineering.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>