irqchip/sifive-plic: Add warning in plic_init() if handler already present

irqchip/sifive-plic: Add warning in plic_init() if handler already present

We have two enteries (one for M-mode and another for S-mode) in the
interrupts-extended DT property of PLIC DT node for each HART. It is
expected that firmware/bootloader will set M-mode HWIRQ line of each
HART to 0xffffffff (i.e. -1) in interrupts-extended DT property
because Linux runs in S-mode only.

If firmware/bootloader is buggy then it will not correctly update
interrupts-extended DT property which might result in a plic_handler
configured twice. This patch adds a warning in plic_init() if a
plic_handler is already marked present. This warning provides us
a hint about incorrectly updated interrupts-extended DT property.

Signed-off-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>