irqchip/sifive-plic: Handle number of hardware interrupts correctly

The driver is handling the number of hardware interrupts inconsistently.

The reason is that the firmware enumerates the maximum number of device
interrupts, but the actual number of hardware interrupts is one more
because hardware interrupt 0 is reserved.

There are two loop variants where this matters:

  1) Iterating over the device interrupts

     for (irq = 1; irq < total_irqs; irq++)

  2) Iterating over the number of interrupt register groups

     for (grp = 0; grp < irq_groups; grp++)

The current code stores the number of device interrupts and that requires
to write the loops as:

  1) for (irq = 1; irq <= device_irqs; irq++)

  2) for (grp = 0; grp < DIV_ROUND_UP(device_irqs + 1); grp++)

But the code gets it wrong all over the place. Just fixing up the
conditions and off by ones is not a sustainable solution as the next changes
will reintroduce the same bugs over and over.

Sanitize it by storing the total number of hardware interrupts during probe
and precalculating the number of groups. To future proof it mark
priv::total_irqs __private, provide a correct iterator macro and adjust the
code to this.

Marking it private allows sparse (C=1 build) to catch direct access to this
member:

  drivers/irqchip/irq-sifive-plic.c:270:9: warning: dereference of noderef expression

That should prevent at least the most obvious future damage in that area.

Fixes: e80f0b6a2cf3 ("irqchip/irq-sifive-plic: Add syscore callbacks for hibernation")
Reported-by: Yangyu Chen <cyy@cyyself.name>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Yangyu Chen <cyy@cyyself.name>
Link: https://patch.msgid.link/87ikcd36i9.ffs@tglx

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