ARM: Tidy up DTB Makefile entries

Few things were out of order:

- removed ARCH_BCM2835 duplicate
- shuffled ARCH_BCM_5301X, ARCH_U8500 and ARCH_U300 around so to keep the
  list sorted

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Olof Johansson <olof@lixom.net>
Signed-by: Domenico Andreoli <domenico.andreoli@linux.com>

Signed-off-by: Arnd Bergmann <arnd@arndb.de>