[PATCH] add dispatch_i8259_irq() to i8259.c

[Jun Sun <jsun@mvista.com>]

If a machine uses i8259 as part of its interrupt routing subsystem,
it typically has two ways to figure out which i8259 interrupt has 
happened.

1) through PCI interrupt ack cycle to directly read the IRQ number
2) read i8259 interrupt status registers and figure which one has
   happened.

This patch adds support for those sorry boards which only have
option 2) available.

While I am here, I also promoted do_IRQ() declaration to a 
header file, as it is needed by all C-level interrupt dispatching
code.

Any feedbacks? 

Jun

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