lib: utils/irqchip: plic: Fix the off-by-one error in plic_context_init()

The number of interrupt enable register in words was once correct,
but was wrongly changed to have an off-by-one error since
commit 8c362e7d ("lib: irqchip/plic: Factor out a context init function").

Fixes: 8c362e7d

 ("lib: irqchip/plic: Factor out a context init function")
Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Samuel Holland <samuel@sholland.org>