omap1: Use asm_irq_flags for entry-macro.S

Initialize asm_irq_flags in omap_init_irq and use it in
get_irqnr_and_base to detect between omap7xx and omap15xx/16xx.

Note that both INT_1510_IH2_IRQ and INT_1510_IH2_IRQ are defined
as 0, so use INT_1510_IH2_IRQ for both of them.

Signed-off-by: Tony Lindgren <tony@atomide.com>