mailbox: OMAP: introduce mailbox framework

mailbox: OMAP: introduce mailbox framework

Actually moving it from plat-omap, as this framework/driver code is
supposed to be under drivers/ folder. The framework should work with
the current supported OMAP processors (OMAP1+) that have mailbox and
can be used as a method of interprocessor communication.

The mailbox hardware (in OMAP) uses a queued mailbox-interrupt mechanism
that provides a communication channel between processors through a set of
registers and their associated interrupt signals by sending and receiving
messages.

As part of the migration from plat and mach code:
- Kconfig symbols have been renamed to build OMAP1 or OMAP2+ drivers.
- mailbox.h has been changed from plat to a dedicated drivers/ folder.

Module names have changed too, instead of mailbox_mach:
- OMAP1: mailbox-omap1.ko
- OMAP2+: mailbox-omap2.ko

Signed-off-by: Omar Ramirez Luna <omar.luna@linaro.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
[s-anna@ti.com: Kconfig fixes for build errors]
Signed-off-by: Suman Anna <s-anna@ti.com>