powerpc: Enable cpuhotplug with ESL=1 on POWER9

[Vaidyanathan Srinivasan]

The attached patch enables ESL=1 STOP2 for cpuhotplug.  This is a debug
patch that we could carry now until STOP states are discovered from
device tree.

Test run:

[  151.670021] CPU8 going offline with request psscr 00000000003f0332
[  151.719856] CPU 8 offline: Remove Rx thread
[  189.200410] CPU8 coming online with psscr 20000000003f0332, srr1 9000000000261001

We request ESL=EC=1 and STOP2 and successfully wakeup from that state
using IPI from XIVE.

--Vaidy