sh: Expose physical addressing mode through cpuinfo.

CPUs can be in either the legacy 29-bit or 32-bit physical addressing
modes. This follows the x86 approach of tracking the phys bits in cpuinfo
and exposing it to userspace through procfs.

This change was requested to permit kexec-tools to detect the physical
addressing mode in order to determine the appropriate address mangling.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>