serial: Rename SERIAL_SUPPORT to SERIAL

Rename these options so that CONFIG_IS_ENABLED can be used with them.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/gardena-smart-gateway-at91sam_defconfig b/configs/gardena-smart-gateway-at91sam_defconfig
index 5946e23..9f9d100 100644
--- a/configs/gardena-smart-gateway-at91sam_defconfig
+++ b/configs/gardena-smart-gateway-at91sam_defconfig
@@ -15,7 +15,7 @@
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="at91sam9g25-gardena-smart-gateway"
 CONFIG_SPL_TEXT_BASE=0x300000
-CONFIG_SPL_SERIAL_SUPPORT=y
+CONFIG_SPL_SERIAL=y
 CONFIG_SPL_SYS_MALLOC_F_LEN=0x1000
 CONFIG_SPL=y
 CONFIG_DEBUG_UART_BOARD_INIT=y