omap2+: Fix unused variable warning for omap_irq_base

Commit 5d190c40
(omap2+: Initialize omap_irq_base for entry-macro.S from
platform code) simplified the handling of omap_irq_base
for multi-omap builds. However, this patch also introduced
a build warning for !MULTI_OMAP2 builds:

arch/arm/mach-omap2/io.c: In function 'omap_irq_base_init':
arch/arm/mach-omap2/io.c:322: warning: unused variable 'omap_irq_base'

Fix this by removing the ifdef. Also simplify things further
by moving omap_irq_base out of entry-macro.S.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
[tony@atomide.com: updated comments]
Signed-off-by: Tony Lindgren <tony@atomide.com>