opal: Don't put the symbol map whole into a property

The huge property trips a bug in some versions of kexec, and it generally
makes looking at the device-tree more painful than it has to be. Instead,
let's just pass the address & size and we'll add a debugfs file on the
Linux side to recover them.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>