[POWERPC] Add bootwrapper function to get virtual reg from the device tree.

This patch adds a new generic device tree processing function that retrieves
virtual reg addresses from the device tree to the bootwrapper code. It also
updates the bootwrapper code to use the new function.

dt_get_virtual_reg() retrieves the virtual reg addresses from the
"virtual-reg" property. If the property can't be found, it uses the "reg"
property and walks the tree to translate it to absolute addresses.

Signed-off-by: Laurent Pinchart <laurentp@cse-semaphore.com>
Acked-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>