IRQCHIP: Update docs regarding irq_domain_add_tree()

Several drivers now use this API, including the ARM GIC driver, so remove
the outdated comment.

Signed-off-by: Kevin Cernekee <[e-mail]>
Cc: [e-mail]
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Cc: [e-mail]
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Patchwork: https://patchwork.linux-mips.org/patch/8839/
Signed-off-by: Ralf Baechle <[e-mail]>

Documentation/IRQ-domain.txt

···
 hwirq number.  The disadvantage is that hwirq to IRQ number lookup is
 dependent on how many entries are in the table.
 
-Very few drivers should need this mapping.  At the moment, powerpc
-iseries is the only user.
+Very few drivers should need this mapping.
 
 ==== No Map ===-
 irq_domain_add_nomap()