target-ppc: Implement bcd_is_valid function

A function to check if all digits of a given BCD number is valid is
here presented because more instructions will need to reuse the
same code.

Signed-off-by: Jose Ricardo Ziviani <joserz@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
