ARM: OMAP: enable mailbox irq per instance

The machine-specific omap2_mbox_startup is called only once
to initialize the whole mbox module, and as a result,
enabling the mbox irq at that point only works for the very first
mailbox instance opened.

Instead, this patch makes sure enable_irq() is called every
time a new mbox instance is opened. In addition, we're now
enabling the mbox's irq only after its notifier_block is registered,
to avoid possible race of receiving an interrupt without invoking
the user's notifier callback.

Signed-off-by: Juan Gutierrez <jgutierrez@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>
[ohad@wizery.com: slightly reworded the commit log]
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>