overlays: i2c1-bcm2708: Don't overwrite i2c1 pins node

It is bad practise to overwrite an entire node in an overlay. Instead,
target the node and overwrite any properties that need changing.

See: https://github.com/raspberrypi/linux/pull/2118



Suggested-by: soodvarun78 <soodvarun78@gmail.com>
Signed-off-by: Phil Elwell <phil@raspberrypi.org>