arm: dts: Improve the bcm27xx inclusion hierarchy

1) The top-level .dts files now include parallel chains of bcm27xx.dtsi
   and bcm27xx-rpi.dtsi files, with no cross-inclusion between the two
   chains.

2) Move definitions and deletions to the point of maximum commonality
   to reduce redundancy.

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