omap: mailbox: convert block api to kfifo

The underlying buffering implementation of mailbox
is converted from block API to kfifo due to the simplicity
and speed of kfifo.

The default size of the kfifo buffer is set to 256 bytes.
This value is configurable at compile time (via
CONFIG_OMAP_MBOX_KFIFO_SIZE), and can be changed at
runtime (via the mbox_kfifo_size module parameter).

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