powerpc: Show processor cache information in sysfs

Collect cache information from the OF device tree and display it in
the cpu hierarchy in sysfs.  This is intended to be compatible at the
userspace level with x86's implementation[1], hence some of the funny
attribute names.  The arrangement of cache info is not immediately
intuitive, but (again) it's for compatibility's sake.

The cache attributes exposed are:

type (Data, Instruction, or Unified)
level (1, 2, 3...)
size
coherency_line_size
number_of_sets
ways_of_associativity

All of these can be derived on platforms that follow the OF PowerPC
Processor binding.  The code "publishes" only those attributes for
which it is able to determine values; attributes for values which
cannot be determined are not created at all.

[1] arch/x86/kernel/cpu/intel_cacheinfo.c

BenH: Turned some printk's into pr_debug, added better NULL checking
in a couple of places.

Signed-off-by: Nathan Lynch <ntl@pobox.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>