target/ppc: Fix wrong number of UAMR register

The SPR UAMR has the number 13, and not 12. (Fortunately it seems like
Linux is not using this register yet - only the privileged version with
number 29 ... that's why nobody noticed this problem yet)

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
