BCM270X_DT: Fix the tinylcd35 overlay RTC support

Now that overlay parameters are applied before the merge (a requirement
for kernel runtime overlays) it is illegal for parameters/overrides to
target nodes in the base DTB. Solve the problem of only enabling I2C
when an RTC option is used by making the RTC fragments conditional,
and including the required status="okay" within the fragments.

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