nocache ram is usually used by DMA to transfer data between
peripherals and ram. Some chips use isolated nocache ram,
which does not necessarily have to be in RAMABLE-REGION.
By specifying the zephyr,nocache-ram options, users can specify
the region where nocache-ram is located. If the user does not
specify it, it defaults to RAMABLE-REGION.
Signed-off-by: Weiwei Guo <guoweiwei@syriusrobotics.com>