target-ppc: Introduce hypervisor call H_GET_TCE

This patch introduces the hypervisor call H_GET_TCE which is basically the
reverse of H_PUT_TCE, as defined in the Power Architecture Platform
Requirements (PAPR).

The hcall H_GET_TCE is required by the kdump kernel which is calling it to
retrieve the TCE set up by the panicing kernel.

Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>