xen/serial: Move any OMAP specific things to OMAP UART driver

xen/serial: Move any OMAP specific things to OMAP UART driver

The 8250-uart.h contains extra serial register definitions
for the internal UARTs in TI OMAP SoCs which are used in
OMAP UART driver only.
In order to clean up code move these definitions to omap-uart.c.
Also rename some definitions to follow to the UART_OMAP* prefix.

Signed-off-by: Oleksandr Tyshchenko <oleksandr.tyshchenko@globallogic.com>
CC: Ian Campbell <ian.campbell@citrix.com>
CC: Julien Grall <julien.grall@citrix.com>
CC: Stefano Stabellini <stefano.stabellini@citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>