cpu-hotplug: Keep atleast 1 online and un-isolated CPU

The PM_QOS_CPU_DMA_LATENCY vote attached to an IRQ is discarded,
if it is affined to an isolated CPU. So we need atleast 1 CPU
in online and un-isolate state. The scheduler rejects isolating
a CPU if it is the only online and un-isolated CPU in the system.
Add the same check for CPU hotplug.

Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
[markivx: forward port to 4.14, fix merge conflicts due to new write-lock]
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>

Change-Id: I16ca44a49dbc7ed385378e08a8634e3c0ac94e03
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>