Currently platforms/drivers needing to get the machine model name are
replicating the same snippet of code. In some case, the OF reference
counting is either missing or incorrect.

This patch adds support to read the machine model name either using
the "model" or the "compatible" property in the device tree root node
to the core OF/DT code.

This can be used to remove all the duplicate code snippets doing exactly
same thing later.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Frank Rowand <frowand.list@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>