Make get_irqnr_and_base common for mach-omap2 multiboot

Thanks to a tip from Russell King <rmk+kernel@arm.linux.org.uk>,
this also optimizes the code for non-multiboot configurations by
using get_irqnr_preamble.

Note that this will only work currently for 24xx and 34xx.
Support for 44xx can be added later on for basic multiboot,
and similar patch should be done for mach-omap1/entry-macro.S.
Signed-off-by: Tony Lindgren <tony@atomide.com>