configs: am335x: am57x: dra7x: Enable CONFIG_SPL_DM_SEQ_ALIAS

Those platforms need CONFIG_SPL_DM_SEQ_ALIAS because they enable both
DM_I2C and SPL_DM. Without CONFIG_SPL_DM_SEQ_ALIAS, it is not possible to
get the I2C bus with i2c_get_chip_for_busnum().

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Felix Brack <fb@ltec.ch>
Tested-by: Felix Brack <fb@ltec.ch>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Heiko Schocher <hs@denx.de>