ARM: footbridge: remove addin mode

This does not appear to have been used in many years, we can
kill off some of the uglier code.

Among other things, it avoids a randconfig issue when both modes
are disabled:

arch/arm/mach-footbridge/common.c:149:24: error: 'ebsa285_host_io_desc' defined but not used [-Werror=unused-variable]
  149 | static struct map_desc ebsa285_host_io_desc[] __initdata = {
      |                        ^~~~~~~~~~~~~~~~~~~~
arch/arm/mach-footbridge/common.c:136:24: error: 'fb_common_io_desc' defined but not used [-Werror=unused-variable]
  136 | static struct map_desc fb_common_io_desc[] __initdata = {
      |                        ^~~~~~~~~~~~~~~~~

The recently added phys_to_dma() functions are now trivial and
could probably be removed again as a follow-up, if anyone knows
how.

Cc: Christoph Hellwig <hch@lst.de>
Tested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>