serial: Add OMAP high-speed UART driver

This patch adds driver support for OMAP2/3/4 high speed UART.

The driver is made separate from 8250 driver as we cannot
over load 8250 driver with omap platform specific configuration for
features like DMA, it makes easier to implement features like DMA and
hardware flow control and software flow control configuration with
this driver as required for the omap-platform.
This patch involves only the core driver and its dependent.

Cc: Tony Lindgren <tony@atomide.com>
Signed-off-by: Govindraj.R <govindraj.raja@ti.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>