lib: irqchip/plic: Disable all contexts and IRQs

To initialize PLIC in sane state, we should:
1. set maximum threshold value of M-mode PLIC contexts
2. set maximum threshold value of S-mode PLIC contexts
3. set irq priorities to miniumum

Fix the comment and initialize the threshold/priorities correctly.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Atish Patra <atish.patra@wdc.com>