powerpc/scom: Improve debugfs interface

The current debugfs interface to scom is essentially unused
and racy. It uses two different files "address" and "data"
to perform accesses which is at best impractical for anything
but manual use by a developer.

This replaces it with an "access" file which represent the entire
scom address space which can be lseek/read/writen too.

This file only supports accesses that are 8 bytes aligned and
multiple of 8 bytes in size. The offset is logically the SCOM
address multiplied by 8.

Since nothing in userspace exploits that file at the moment, the ABI
change is a no-brainer.

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