target/ppc/POWER9: Add cpu_has_work function for POWER9

target/ppc/POWER9: Add cpu_has_work function for POWER9

The cpu has work function is used to mask interrupts used to determine
if there is work for the cpu based on the LPCR. Add a function to do this
for POWER9 and add it to the POWER9 cpu definition. This is similar to that
for POWER8 except using the LPCR bits as defined for POWER9.

Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>