serial: omap: remove unnecessary header and add a missing one

this driver doesn't use any from <plat/dmtimer.h>, so
we can remove it without any problems.

This will, however cause a problem because omap-serial.c
was relying on indirect inclusion of <linux/platform_device.h>,
let's fix the issue by including <linux/platform_device.h>
on omap-serial.c as it should be.
Tested-by: NShubhrajyoti D <shubhrajyoti@ti.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/omap-serial.c

@@ -32,6 +32,7 @@
 #include <linux/slab.h>
 #include <linux/tty.h>
 #include <linux/tty_flip.h>
+#include <linux/platform_device.h>
 #include <linux/io.h>
 #include <linux/clk.h>
 #include <linux/serial_core.h>
@@ -40,7 +41,6 @@
 #include <linux/of.h>
 #include <linux/gpio.h>
 
-#include <plat/dmtimer.h>
 #include <plat/omap-serial.h>
 
 #define UART_BUILD_REVISION(x, y)	(((x) << 8) | (y))