ARM: dts: imx6: fix dtc warnings for ipu endpoints

When compiled with "W=1", dtc complains: e.g.
"Warning (unit_address_vs_reg):
Node /soc/ipu@02800000/port@2/endpoint@0
has a unit name, but no reg property"

Endpoint nodes don't have a reg property, and the addresses
in their node names are ordinals without any special meaning
so remove them and swap them for semantic node names.

Signed-off-by: Joshua Clayton <stillcompiling@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>