i2c: Mark instantiated device nodes with OF_POPULATE

Mark (and unmark) device nodes with the POPULATE flag as appropriate.
This is required to avoid multi probing when using I2C and device
overlays containing a mux.
This patch is also more careful with the release of the adapter device
which caused a deadlock with muxes, and does not break the build
on !OF since the node flag accessors are not defined then.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>