spl: Add dependency on serial to Ymodem

Building with Ymodem support requires serial in SPL/TPL, add that
dependency here.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
diff --git a/common/spl/Kconfig b/common/spl/Kconfig
index 4d27565..c5d4b5e 100644
--- a/common/spl/Kconfig
+++ b/common/spl/Kconfig
@@ -751,6 +751,7 @@
 
 config SPL_YMODEM_SUPPORT
 	bool "Support loading using Ymodem"
+	depends on SPL_SERIAL_SUPPORT
 	help
 	  While loading from serial is slow it can be a useful backup when
 	  there is no other option. The Ymodem protocol provides a reliable
@@ -951,6 +952,7 @@
 
 config TPL_YMODEM_SUPPORT
 	bool "Support loading using Ymodem"
+	depends on TPL_SERIAL_SUPPORT
 	help
 	  While loading from serial is slow it can be a useful backup when
 	  there is no other option. The Ymodem protocol provides a reliable