ti: keystone: Don't select GPIO_EXTRA_HEADER

keystone doesn't have custom gpio.h that's why don't select
GPIO_EXTRA_HEADER which points to it.

Logic in arch/arm/include/asm/gpio.h is very clear

 #ifdef CONFIG_GPIO_EXTRA_HEADER
 #include <asm/arch/gpio.h>
 #endif
 #include <asm-generic/gpio.h>

Where it is visible that there is no gpio.h in platform headers:
$ ls arch/arm/mach-keystone/include/mach/
clock_defs.h  clock-k2e.h  clock-k2hk.h  ddr3.h      hardware-k2e.h
hardware-k2hk.h  i2c_defs.h      mon.h   mux-k2g.h   xhci-keystone.h
clock.h       clock-k2g.h  clock-k2l.h   hardware.h  hardware-k2g.h
hardware-k2l.h   mmc_host_def.h  msmc.h  psc_defs.h

Signed-off-by: Michal Simek <michal.simek@amd.com>
Reviewed-by: Tom Rini <trini@konsulko.com>