omap: mailbox: convert rwlocks to spinlock

rwlocks are slower and have potential starvation issues
therefore spinlocks are generally preferred.

see also: http://lwn.net/Articles/364583/



Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Signed-off-by: Kanigeri Hari <h-kanigeri2@ti.com>
Signed-off-by: Hiroshi DOYU <Hiroshi.DOYU@nokia.com>