powerpc/powernv: Rename idle_power7.S to idle_book3s.S

idle_power7.S handles idle entry/exit for POWER7, POWER8 and in next
patch for POWER9. Rename the file to a non-hardware specific
name.

Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Shreyas B. Prabhu <shreyas@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>