OMAP4 Mailbox driver - Includes Hiroshi's changes on mailbox driver along with API changes introducted in the 2.6.31 kernel

This patch cleans up the mailbox driver module and adds the
following changes:
	- Remove sequence bit checking (protocol logic not
		required in the driver)
	- Remove class interface (not used presently)
	- Remove disalbe_/enable_mbox_irq in isr (No irqs
		happen in the isr)
	- Remove unnecessary arg for omap_mbox_msg_send
	- Expose omap_mbox_enable()/disable_irq()

NOTE::
For OMAP4 the tx work queue does not ssem to function
properly. Hence, calling __mbox_msg_send for now.
Signed-off-by: C A Subramaniam <subramaniam.ca@ti.com>
Signed-off-by: Ramesh Gupta G <grgupta@ti.com>