ARM: initial multiplatform support

This lets us build a multiplatform kernel for experimental purposes.
However, it will not be useful for any real work, because it relies
on a number of useful things to be disabled for now:

* SMP support must be turned off because of conflicting symbols.
  Marc Zyngier has proposed a solution by adding a new SOC
  operations structure to hold indirect function pointers
  for these, but that work is currently stalled

* We turn on SPARSE_IRQ unconditionally, which is not supported
  on most platforms. Each of them is currently in a different
  state, but most are being worked on.

* A common clock framework is in place since v3.4 but not yet
  being used. Work on this is on its way.

* DEBUG_LL for early debugging is currently disabled.

* THUMB2_KERNEL does not work with allyesconfig because the
  kernel gets too big

[Rob Herring]: Rebased to not be dependent on the mass mach header rename.
As a result, omap2plus, imx, mxs and ux500 are not converted. Highbank,
picoxcell, mvebu, and socfpga are converted.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Andrew Lunn <andrew@lunn.ch>
Acked-by: Jamie Iles <jamie@jamieiles.com>
Cc: Dinh Nguyen <dinguyen@altera.com>