edac: sb_edac.c should not require prescence of IMC_DDRIO device

The Sandy Bridge EDAC driver uses a register in the IMC_DDRIO CSR
space to determine the type of DIMMs (registered or unregistered).
But this device does not exist on some single socket Sandy Bridge
servers.  While the type of DIMMs is nice to know, it is not essential
for this driver's other functions. So it seems harsh to have it
refuse to load at all when it cannot find this device.

Make the check for this device be optional. If it isn't present
just report the memory type as "MEM_UNKNOWN".

Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>