add a not device managed memremap_pages v2

[Christoph Hellwig]

Hi Dan and Jason,

Bharata has been working on secure page management for kvmppc guests,
and one I thing I noticed is that he had to fake up a struct device
just so that it could be passed to the devm_memremap_pages
instrastructure for device private memory.

This series adds non-device managed versions of the
devm_request_free_mem_region and devm_memremap_pages functions for
his use case.

Changes since v1:
 - don't overload devm_request_free_mem_region
 - export the memremap_pages and munmap_pages as kvmppc can be a module