[POWERPC] EEH: support MMIO enable recovery step

Update to the PowerPC PCI error recovery code.

Add code to enable MMIO if a device driver reports that it is capable
of recovering on its own.  One anticipated use of this having a device
driver enable MMIO so that it can take a register dump, which might
then be followed by the device driver requesting a full reset.

Signed-off-by: Linas Vepstas <linas@austin.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>