arm: dts: ti: omap: am5729-beagleboneai: Drop the OPP

opp_slow is not defined in the table in dra7 or derivatives, drop the
definition.

Signed-off-by: Nishanth Menon <nm@ti.com>
Message-ID: <20230724153911.1376830-3-nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>