rpi3: Move rpi3_hw.h header file to include/rpi_hw.h

With the advent of Raspberry Pi 4 support, we need to separate some
board specific headers between the RPi3 and RPi4.
Rename and move the "rpi3_hw.h" header, so that .c files just include
rpi_hw.h, and automatically get the correct version.

Change-Id: I03b39063028d2bee1429bffccde71dddfe2dcde8
Signed-off-by: Andre Przywara <andre.przywara@arm.com>