rockchip/rk3399: Select GENERIC_GPIO_LIB Kconfig

This option should've been selected by RK3399 the whole time since the
SoC supports the <soc/gpio.h> interface. It wasn't really a big deal
until now where I'm trying to use a the base2 read helper, though.

Change-Id: Ib7a5f00a6680163105fc0598ce77d03f3645f05a
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/22744
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/rockchip/rk3399/Kconfig

@@ -8,6 +8,7 @@ config SOC_ROCKCHIP_RK3399
 	select ARM64_USE_ARM_TRUSTED_FIRMWARE
 	select BOOTBLOCK_CONSOLE
 	select DRIVERS_UART_8250MEM_32
+	select GENERIC_GPIO_LIB
 	select GENERIC_UDELAY
 	select HAVE_MONOTONIC_TIMER
 	select UART_OVERRIDE_REFCLK