blkmap: add an attribute to preserve the mem mapping

blkmap: add an attribute to preserve the mem mapping

Some blkmap memory mapped devices might have to be relevant even
after U-Boot passes control to the next image as part of the platform
boot. An example of such a mapping would be an OS installer ISO image,
information for which has to be provided to the OS kernel. Use the
'preserve' attribute for such mappings. The code for adding a pmem
node to the device-tree then checks if this attribute is set, and adds
a node only for mappings which have this attribute.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>