Exynos SoC model

The "cpu-model" property is a non-standard extension for the device tree root
node. Since the cpu id of some Exynos variants does not correspond to product
name, this property fills the gap.

For almost all Exynos based boards in the kernel, the product name corresponds
to the device tree file name. The same name is generated in U-Boot, so the new
property allows doing it automatically.

Required properties:
 - cpu-model : Exynos product name

Example:

/ {
	model = "Samsung/Google Peach Pi board based on Exynos5800";
	cpu-model = "Exynos5800";

	compatible = ...
};