mambo: Merge PMEM_DISK and PMEM_VOLATILE code
mambo: Merge PMEM_DISK and PMEM_VOLATILE code

PMEM_VOLATILE and PMEM_DISK can't be used together and are basically
copies of the same code.

This merges the two and allows them used together.  Same API is kept.

Signed-off-by: Michael Neuling <7b0ab45a730e48a69239010b5b8fe5fa4e8eaac6@neuling.org>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.ibm.com>

legoater/skiboot,shenki/skiboot,qemu/skiboot,shenki/skiboot,open-power/skiboot,legoater/skiboot,stewart-ibm/skiboot,open-power/skiboot,legoater/skiboot,qemu/skiboot,qemu/skiboot,open-power/skiboot,shenki/skiboot,open-power/skiboot,stewart-ibm/skiboot,qemu/skiboot,shenki/skiboot,qemu/skiboot,legoater/skiboot,legoater/skiboot,shenki/skiboot,stewart-ibm/skiboot,open-power/skiboot