powerpc/powernv: Provide debugfs access to the LPC bus via OPAL

This provides debugfs files to access the LPC bus on Power8
non-virtualized using the appropriate OPAL firmware calls.

The usage is simple: one file per space (IO, MEM and FW),
lseek to the address and read/write the data. IO and MEM always
generate series of byte accesses. FW can generate word and dword
accesses if aligned properly.

Based on an original patch from Rob Lippert and reworked.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>