board: ti: am43xx_evm_usbboot: Enable DM for USB, fix SPL build errors

To address the following warning message:

===================== WARNING ======================
This board does not use CONFIG_DM_USB. Please update
the board to use CONFIG_DM_USB before the v2019.07 release.
Failure to update by the deadline may result in board removal.
See doc/driver-model/MIGRATION.txt for more info.
====================================================

CONFIG_DM_USB is enabled, this resulted in SPL build errors:

drivers/built-in.o: In function 'xhci_dwc3_probe':
u-boot/drivers/usb/host/xhci-dwc3.c:155: undefined reference to 'usb_get_dr_mode'
scripts/Makefile.spl:404: recipe for target 'spl/u-boot-spl' failed
make[1]: *** [spl/u-boot-spl] Error 1
Makefile:1721: recipe for target 'spl/u-boot-spl' failed
make: *** [spl/u-boot-spl] Error 2

Enabling usb common library and usb ethernet drivers in SPL
does the job. Target was compile tested, build was clean.

Signed-off-by: Suniel Mahesh <sunil.m@techveda.org>

configs/am43xx_evm_usbhost_boot_defconfig

@@ -15,6 +15,8 @@ CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL_MTD_SUPPORT=y
 CONFIG_SPL_OS_BOOT=y
+CONFIG_SPL_USB_GADGET=y
+CONFIG_SPL_USB_ETHER=y
 CONFIG_SPL_USB_HOST_SUPPORT=y
 CONFIG_CMD_SPL=y
 CONFIG_CMD_SPL_NAND_OFS=0x00100000
@@ -60,6 +62,7 @@ CONFIG_TI_QSPI=y
 CONFIG_TIMER=y
 CONFIG_OMAP_TIMER=y
 CONFIG_USB=y
+CONFIG_DM_USB=y
 CONFIG_USB_XHCI_HCD=y
 CONFIG_USB_XHCI_DWC3=y
 CONFIG_USB_DWC3=y