overlays: tc358743: Use intra-overlay fragments

The tc358743 overlay was written using a workaround to a problem with
fragments that target other fragments, but this had the unfortunate
side-effect of preventing the overlay from being applied at runtime
(the kernel doesn't allow nodes to be overwritten by an overlay, only
properties).

The current firmware and dtoverlay/dtparam utilities include support
for these "intra-overlay" fragments, so remove the workaround and do
it properly.

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