mfd: Fix compilation error in tps65910.h

'struct gpio_chip' is declared in include/asm-generic/gpio.h
which is included by include/linux/gpio.h.
However without including gpio.h, TPS65910.h declares
a member of this type as part of 'struct tps65910' declaration.

This causes compilation error, if gpio.h is not included
before including tps65910.h, in source files.

Signed-off-by: Venu Byravarasu <vbyravarasu@nvidia.com>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>