lib: utils/irqchip: Add FDT wrappers for PLIC save/restore functions

These functions save/restore the state of the PLIC associated with the
current hart. The context save/restore functions only manipulate a
single context, since most likely the M-mode context is unused and does
not need to be saved.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Anup Patel <anup@brainfault.org>