dm: at91: snapper: 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>

configs/snapper9260_defconfig

 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9260"
 CONFIG_ARM=y
 CONFIG_TARGET_SNAPPER9260=y
+CONFIG_DM=y
+CONFIG_DM_GPIO=y
+CONFIG_DM_SERIAL=y

configs/snapper9g20_defconfig

 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9G20"
 CONFIG_ARM=y
 CONFIG_TARGET_SNAPPER9260=y
+CONFIG_DM=y
+CONFIG_DM_GPIO=y
+CONFIG_DM_SERIAL=y

include/configs/snapper9260.h

@@ -21,10 +21,6 @@
 #define CONFIG_SYS_AT91_MAIN_CLOCK	18432000 /* External Crystal, in Hz */
 #define CONFIG_SYS_AT91_SLOW_CLOCK	32768
 #define CONFIG_SYS_GENERIC_BOARD
-#define CONFIG_DM
-#define CONFIG_CMD_DM
-#define CONFIG_DM_GPIO
-#define CONFIG_DM_SERIAL
 #define CONFIG_SYS_MALLOC_F_LEN		(1 << 10)
 
 /* CPU */