spl: Add SPL_SERIAL as requirement for SDP_USB_SDP

The USB SDP protocol require the SPL serial support to allow the build
to succeed.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
diff --git a/common/spl/Kconfig b/common/spl/Kconfig
index 807b1dc..620ea1e 100644
--- a/common/spl/Kconfig
+++ b/common/spl/Kconfig
@@ -1237,6 +1237,7 @@
 
 config SPL_USB_SDP_SUPPORT
 	bool "Support SDP (Serial Download Protocol)"
+	depends on SPL_SERIAL_SUPPORT
 	help
 	  Enable Serial Download Protocol (SDP) device support in SPL. This
 	  allows to download images into memory and execute (jump to) them