configs: am335x_boneblack_vboot_defconfig: Add DM for SPI and Flash devices

This patch adds SPI and SPI_FLASH DM support for verified boot on
TI AM335 chipsets. The following compile warning is removed:

===================== WARNING ======================
This board does not use CONFIG_DM_SPI_FLASH. Please update
the board to use CONFIG_SPI_FLASH 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.
====================================================

Built and tested on AM335x device (BeagleboneBlack).

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