rockchip: rk3288: vyasa: Drop ROCKCHIP_BROM_HELPER selection

ROCKCHIP_BROM_HELPER is selected as if TPL/SPL ROCKCHIP_BACK_TO_BROM
has been defined, so drop the explicit enablement for vyasa board.

This change is supposed to missed during config move to
other locations, and missed to drop the same.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever <Yang&lt;kever.yang@rock-chips.com>