powerpc/pseries: Move CMO code from plapr_wrappers.h to platforms/pseries

Currently there's some CMO (Cooperative Memory Overcommit) code, in
plpar_wrappers.h. Some of it is #ifdef CONFIG_PSERIES and some of it
isn't. The end result being if a file includes plpar_wrappers.h it won't
build with CONFIG_PSERIES=n.

Fix it by moving the CMO code into platforms/pseries. The two hcall
wrappers can just be moved into their only caller, cmm.c, and the
accessors can go in pseries.h.

Note we need the accessors because cmm.c can be built as a module, so
there needs to be a split between the built-in code vs the module, and
that's achieved by using those accessors.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>