rockchip: rk3399: include gpio.h

rockchip: rk3399: include gpio.h

After applying the series for debug_uart_init(), Travis-CI reports:

arch/arm/mach-rockchip/rk3399/rk3399.c:90:2: error: implicit declaration of function 'spl_gpio_set_pull' [-Werror=implicit-function-declaration]
  spl_gpio_set_pull(&pmugrf->gpio0_p, GPIO(BANK_B, 2), GPIO_PULL_NORMAL);
  ^~~~~~~~~~~~~~~~~

This is caused by a missing header-file include.  Fix it.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>