powerpc/pseries: Export raw per-CPU VPA data via debugfs

powerpc/pseries: Export raw per-CPU VPA data via debugfs

[ Upstream commit c6c26fb55e8e4b3fc376be5611685990a17de27a ]

This patch exports the raw per-CPU VPA data via debugfs.
A per-CPU file is created which exports the VPA data of
that CPU to help debug some of the VPA related issues or
to analyze the per-CPU VPA related statistics.

v3: Removed offline CPU check.

v2: Included offline CPU check and other review comments.

Signed-off-by: Aravinda Prasad <aravinda@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Sasha Levin <sashal@kernel.org>