Kconfig: CONFIG_OF_PLATDATA doesn't really exist

There is no CONFIG_OF_PLATDATA, only CONFIG_SPL_OF_PLATDATA, so rename
the two references to CONFIG_OF_PLATDATA to CONFIG_SPL_OF_PLATDATA.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index b26ada3..b11f3ff 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -352,7 +352,7 @@
 	depends on DM_SERIAL && SPL_OF_PLATDATA
 	help
 	  Select this to enable a debug UART for Rockchip devices when using
-	  CONFIG_OF_PLATDATA (i.e. a compiled-in device tree replacemenmt).
+	  CONFIG_SPL_OF_PLATDATA (i.e. a compiled-in device tree replacemenmt).
 	  This uses the ns16550 driver, converting the platdata from of-platdata
 	  to the ns16550 format.