This adds support for const properties. We add a const void *value_const
to struct property which will not be modified. The value of a property
should no longer be used directly, but with the of_property_get_value()
accessor which picks the right value.
With this we can unflatten dtbs and use the property values directly
from the dtb instead of copying them. This is useful for device trees
which (ab)use properties to store huge data files, aka FIT images.
To create a property whose value is not copied but used from the
original input data we introduce of_new_property_const().

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>