powerpc/eeh_cache: Add a way to dump the EEH address cache

Adds a debugfs file that can be read to view the contents of the EEH
address cache. This is pretty similar to the existing
eeh_addr_cache_print() function, but that function is intended to debug
issues inside of the kernel since it's #ifdef`ed out by default, and writes
into the kernel log.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Reviewed-by: Sam Bobroff <sbobroff@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>