lib: irqchip/plic: Fix maximum priority threshold value

As per the PLIC specification, maximum priority threshold value is 0x7.
Even though, writing a higher value doesn't cause any error in qemu
hifive unleashed, there may be some implementation which checks the upper
and may result in an illegal access.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>