powerpc/powernv: Move IDLE_STATE_ENTER_SEQ macro to cpuidle.h

Move IDLE_STATE_ENTER_SEQ macro to cpuidle.h so that MCE handler changes
in subsequent patch can use it.

No functionality change.

Signed-off-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>