ARM: imx6ul: Add support for liteSOM

liteSOM is a System On Module (http://grinn-global.com/litesom/

). It
can't exists on its own, but will be used as part of other boards.

Hardware specification:
 * NXP i.MX6UL processor
 * 256M or 512M DDR3 memory
 * optional eMMC (uSDHC2)

Here we treat SOM similar to SOC, so we place it inside arch/arm/mach-*
directory and make it possible to reuse initialization code (i.e. DDR,
eMMC init) for all boards that use it.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>