arch: common: Add user can specify the nocache location

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>