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>
diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index 236debb..ef4f69d 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -58,6 +58,7 @@
 machine-$(CONFIG_ARCH_KEYSTONE)		+= keystone
 # TODO: rename CONFIG_KIRKWOOD -> CONFIG_ARCH_KIRKWOOD
 machine-$(CONFIG_KIRKWOOD)		+= kirkwood
+machine-$(CONFIG_LITESOM)		+= litesom
 machine-$(CONFIG_ARCH_MESON)		+= meson
 machine-$(CONFIG_ARCH_MVEBU)		+= mvebu
 # TODO: rename CONFIG_TEGRA -> CONFIG_ARCH_TEGRA