powerpc/nvdimm: Add support for multibyte read/write for metadata

SCM_READ/WRITE_MEATADATA hcall supports multibyte read/write. This patch
updates the metadata read/write to use 1, 2, 4 or 8 byte read/write as
mentioned in PAPR document.

READ/WRITE_METADATA hcall supports the 1, 2, 4, or 8 bytes read/write.
For other values hcall results H_P3.

Hypervisor stores the metadata contents in big-endian format and in-order
to enable read/write in different granularity, we need to switch the contents
to big-endian before calling HCALL.

Based on an patch from Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>