fdt: add support for adding pmem nodes

fdt: add support for adding pmem nodes

One of the problems an OS may face, when running in EFI, is that
a mounted ISO, after calling ExitBootServices goes away, if that ISO
is resident in RAM memory as a ramdisk.

ACPI has NFIT and NVDIMM support to provide ramdisks to the OS, but we
don't have anything in place for DTs. Linux and device trees have support
for persistent memory devices. So add a function that can inject a pmem
node in a DT, so we can pass information on the ramdisk the OS.

Signed-off-by: Masahisa Kojima <kojima.masahisa@socionext.com>
Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>