dm: stv0991: Move driver model CONFIGs to Kconfig

Remove driver model CONFIGs from the board config headers and use Kconfig
instead.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/stv0991_defconfig b/configs/stv0991_defconfig
index 37b10ba..1c9ba88 100644
--- a/configs/stv0991_defconfig
+++ b/configs/stv0991_defconfig
@@ -3,3 +3,5 @@
 CONFIG_TARGET_STV0991=y
 CONFIG_SYS_MALLOC_F=y
 CONFIG_SYS_MALLOC_F_LEN=0x2000
+CONFIG_DM=y
+CONFIG_DM_SERIAL=y