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

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>