flash/nor/xmc4xxx: Loosen checks for XMC4500

According to Infineon, XMC4500 EES AA13 with date codes before GE212 -
as seen on an XMC4500 General App Kit - had a zero SCU_IDCHIP register.

Handle this by extending our checks to not error out on zero SCU_IDCHIP
and by printing a useful info string in that case.

Change-Id: Ic2d641a314627dd5a1ff775a0113999191b95e3d
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-on: http://openocd.zylin.com/2751


Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-by: Jeff Ciesielski <jeffciesielski@gmail.com>