ARM: exynos: enable multiplatform support

This makes it possible to enable the exynos platform as part of a
multiplatform kernel, in addition to keeping the single-platform
exynos support.

The multiplatform variant has a number of limitations at the moment:

* It only supports DT-enabled machines. This is not a problem in
  the long run, as non-DT machines for exynos are going away.
  The main problem here is that the gpio code and the exynos_eint
  irqchip are not multiplatform capable but still required for
  ATAGS based boot.
* The watchdog driver is still missing a conversion.
* sparsemem and memory_holes are currently not supported in
  multiplatform.

The the multiplatform aware ARCH_EXYNOS Kconfig symbol is disabled
for now, as dependent patches are still pending in other
subsystem trees. We will enable it once everything comes together.

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