ARM: Allow ARCH_MULTIPLATFORM to be selected for NOMMU

With this patch applied potentially any platform can be built in NOMMU
configurations if CONFIG_EXPERT is selected. However, there is no
guaranty that platform can successfully run such Image. So the main
motivation behind of this patch:
- bring build coverage for NOMMU configurations
- allow known working NOMMU platforms (like R-class) to be used
- pave a way to add support for single address space (aka 1:1 mapping)
  for MMU platforms, so they can be usable in NOMMU configurations

Cc: Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ryan Mallon <rmallon@gmail.com>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Alexander Shiyan <shc_work@mail.ru>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Russell King <linux@armlinux.org.uk>
Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>