powerpc/papr_scm: Implement support for H_SCM_FLUSH hcall

Add support for ND_REGION_ASYNC capability if the device tree
indicates 'ibm,hcall-flush-required' property in the NVDIMM node.
Flush is done by issuing H_SCM_FLUSH hcall to the hypervisor.

If the flush request failed, the hypervisor is expected to
to reflect the problem in the subsequent nvdimm H_SCM_HEALTH call.

This patch prevents mmap of namespaces with MAP_SYNC flag if the
nvdimm requires an explicit flush[1].

References:
[1] https://github.com/avocado-framework-tests/avocado-misc-tests/blob/master/memory/ndctl.py.data/map_sync.c

Signed-off-by: Shivaprasad G Bhat <[email protected]>
Reviewed-by: Aneesh Kumar K.V <[email protected]>
[mpe: Use unsigned long / long instead of uint64_t/int64_t]
Signed-off-by: Michael Ellerman <[email protected]>
Link: https://lore.kernel.org/r/161703936121.36.7260632399582101498.stgit@e1fbed493c87

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