Directed Hypervisor Doorbell Interrupts come in at 0xe80 (or
0xc000000000004e80 if relocation on exceptions is enabled), so add
exception vectors at these locations.

If doorbell support is not compiled in we handle it as an
unknown_exception.
Signed-off-by: NIan Munsie <imunsie@au1.ibm.com>
Tested-by: NMichael Neuling <mikey@neuling.org>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>