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the outdated comment.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 <cernekee@gmail.com>
Cc: f.fainelli@gmail.com
Cc: jaedon.shin@gmail.com
Cc: abrestic@chromium.org
Cc: tglx@linutronix.de
Cc: jason@lakedaemon.net
Cc: jogo@openwrt.org
Cc: computersforpeace@gmail.com
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/8839/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

Documentation/IRQ-domain.txt

@@ -95,8 +95,7 @@ since it doesn't need to allocate a table as large as the largest
 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()