rockchip: rk3399: fix incorrect ifdef check on SPL_GPIO

The check to perform is on CONFIG_SPL_GPIO and not SPL_GPIO.
Because this was never compiled in, it missed an include of cru.h that
was not detected before. Let's include it too.

Also switch to IS_ENABLED in-code check as it is the preferred
inclusion/exclusion mechanism.

Fixes: 07586ee4 ("rockchip: rk3399: Support common spl_board_init")
Cc: Quentin Schulz <foss+uboot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>