ARM: OMAP2: Add missing include of linux/gpio.h

I got some build error like below while executing "make omap2plus_defconfig".

  CC      arch/arm/mach-omap2/board-2430sdp.o
arch/arm/mach-omap2/board-2430sdp.c: In function 'omap_2430sdp_init':
arch/arm/mach-omap2/board-2430sdp.c:247: error: 'GPIOF_OUT_INIT_LOW' undeclared (first use in this function)
arch/arm/mach-omap2/board-2430sdp.c:247: error: (Each undeclared identifier is reported only once
arch/arm/mach-omap2/board-2430sdp.c:247: error: for each function it appears in.)

This patch fixes the build error by include linux/gpio.h instead of mach/gpio.h.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Cc: Syed Mohammed Khasim <x0khasim@ti.com>
Cc: Grazvydas Ignotas <notasas@gmail.com>
Cc: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>

arch/arm/mach-omap2/board-2430sdp.c

@@ -26,13 +26,13 @@
 #include <linux/err.h>
 #include <linux/clk.h>
 #include <linux/io.h>
+#include <linux/gpio.h>
 
 #include <mach/hardware.h>
 #include <asm/mach-types.h>
 #include <asm/mach/arch.h>
 #include <asm/mach/map.h>
 
-#include <mach/gpio.h>
 #include <plat/board.h>
 #include <plat/common.h>
 #include <plat/gpmc.h>