overlays: Support i2c3-6 in container overlays

The I2C container overlays (i2c-rtc, i2c-fan and now i2c-sensor)
take parameters to select which I2C bus to use. Extend the list of
busses to include I2C3 to I2C6.

Note that this has been possible in a way that does prevent the overlay
from loading on older Pis due to the use of target-path = "i2c3"
instead of target = <&i2c3>. The latter fails on a non-BCM2711 Pi when
the overlay is loaded, whereas the former only fails when the fragment
in question is applied (and not at all if the fragment is dormant, or
path is created by a parameter that is not used).

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