ARM: AM43xx: Fix mmcboot command in EXTRA_ENV_SETTINGS

loadbootenv expects devtype variable to be set. This is missing in
mmcboot command. With this the following error comes:
U-Boot# run mmcboot
mmc0 is current device
SD/MMC found on device 0
** Bad device usb 0 **
** Bad device usb 0 **
Fixing this by setting devtype as mmc.

Reported-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>

include/configs/am43xx_evm.h

@@ -179,6 +179,7 @@
 	"loadfdt=load ${devtype} ${bootpart} ${fdtaddr} ${bootdir}/${fdtfile}\0" \
 	"mmcboot=mmc dev ${mmcdev}; " \
 		"setenv devnum ${mmcdev}; " \
+		"setenv devtype mmc; " \
 		"if mmc rescan; then " \
 			"echo SD/MMC found on device ${devnum};" \
 			"if run loadbootenv; then " \