OMAP: mailbox: fix detection for previously supported chips

Fix the mailbox detection for OMAP3630 and 2430, also minor
cleanup on conditional ifdef's that could affect it.

Given that 2430 has an iva too, include it, as the same steps
for omap3 apply.

Signed-off-by: Omar Ramirez Luna <omar.ramirez@ti.com>
Acked-by: Hiroshi Doyu <hiroshi.doyu@nokia.com>