irqchip: intc-irqpin: Add force comments

Add comments to describe the special case for
"force" versions of enable and disable functions.

Signed-off-by: Magnus Damm <damm@opensource.se>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>