Add dependencies for MALLOC_F and OF_LIBFDT

Some features implicitly depended on MALLOC_F and OF_LIBFDT and would
fail at link-time if these features were not enabled.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index ece7d87..bd95f70 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -100,6 +100,7 @@
 config DM_SERIAL
 	bool "Enable Driver Model for serial drivers"
 	depends on DM
+	select SYS_MALLOC_F
 	help
 	  Enable driver model for serial. This replaces
 	  drivers/serial/serial.c with the serial uclass, which
@@ -136,6 +137,7 @@
 config SPL_DM_SERIAL
 	bool "Enable Driver Model for serial drivers in SPL"
 	depends on DM_SERIAL && SPL_DM
+	select SYS_SPL_MALLOC_F
 	default y
 	help
 	  Enable driver model for serial in SPL. This replaces
@@ -146,6 +148,7 @@
 config TPL_DM_SERIAL
 	bool "Enable Driver Model for serial drivers in TPL"
 	depends on DM_SERIAL && TPL_DM
+	select SYS_TPL_MALLOC_F
 	default y if TPL && DM_SERIAL
 	help
 	  Enable driver model for serial in TPL. This replaces