powerpc/xmon: Dump all 64 bits of HDEC

ISA 3.0 defines hypervisor decrementer to be 64 bits in length.
This patch extends the print format for to be 64 bits.

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