mailbox/omap: simplify the fifo assignment by using macros

The OMAP mailbox IP has two different type of interrupt configuration
registers between OMAP4+ SoCs and OMAP2/3 SoCs. Simplify the current
interrupt configuration by using a single macro that translates the
two variants.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>