configs: am43xx_evm_qspiboot_defconfig: Move to DM

Move am43xx_evm_qspiboot_defconfig to DM. This is required as SPI core
and TI QSPI driver no longer supports non DM interfaces.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>