ARM: ux500: Create a DTS hierarchy for ST-Ericsson's HREF boards

There are two main versions of the ST-Ericsson hardware reference board,
each with subtle differences. These versions are described in current
platform data as 'mop500' for HREF v1.1 & v2.0 and 'hrefv60+' for latter
incarnations. However, the boards have much in common, so this patch
creates a .dtsi (Device Tree Source Include) file which describes the
similarities between the two boards and individual 'hrefprev60' and
'hrefv60plus' .dts files which depict the differences.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>