arc: add ECR (exception cause register) output

Exception cause register (ECR) contains value that describes a reason
for exception that has happened. This helps a lot to figure-out what
went wrong.

Now we print this register contents when dumping registers.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Igor Guryanov <guryanov@synopsys.com>