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>