papr SCM driver fixes

[Oliver O'Halloran]

Various bug fixes for the papr_scm driver that were found while bringing
up the PowerVM implementation of the interface. There's a few minor bugs
there were a result of bugs in the original QEMU implementation, a few
due to the memory layouts being different and one due to a change to the
DT bindings.

Mostly self-contained to the driver with the exception of the last patch
which touches the powerpc specific parts of the vmemmap handling.