[TF-M] [RFC] Core: Set PendSV priority to lowest

Hi,

I have created a patch to set PenSV priority as lowest, which makes more sense that other high priority interrupts may preempt scheduling and affect the scheduling result. The issue is created here and you can find gerrit link in comment: https://developer.trustedfirmware.org/T310

Please publish your comments under this issue if you have.

Thanks

-Ken