ARM: bcm2835: convert to multi-platform

This allows BCM2835 be included in a kernel build that supports multiple
SoCs at once, which is useful for distro kernels.

This change:
* Moves bcm2835's debug-macro.S into ARM's include/debug/, and hooks it
  into the relevant menu.
* Moves bcm2835's Kconfig into its own directory, as seems typical for
  multi-platform conversions.
* Removes bcm2835_soc.h, and moves the content to the files where it was
  used; just one usage per define.
* Deletes some headers and Makefile.boot that aren't needed now that we
  support multi-platform.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>