Implement H_SET_MODE for ppc64le

Use the infrastructure for queuing a task to a specific vCPU and
sett ILE (Little Endian Interrupt Handling) on power via h_set_mode
hypercall

Signed-off-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Will Deacon <will.deacon@arm.com>