rockchip: serial: Kconfig: add select SYS_NS16550 to config ROCKCHIP_SERIAL

The Rockchip serial driver depends on an enabled NS16550 driver,
so add select SYS_NS16550 to config ROCKCHIP_SERIAL.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index dc514c9..b5c6005 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -763,6 +763,7 @@
 config ROCKCHIP_SERIAL
 	bool "Rockchip on-chip UART support"
 	depends on DM_SERIAL && SPL_OF_PLATDATA
+	select SYS_NS16550
 	help
 	  Select this to enable a debug UART for Rockchip devices when using
 	  CONFIG_SPL_OF_PLATDATA (i.e. a compiled-in device tree replacemenmt).