board: ti: AM43XX: Add ddr voltage rail configuration

Add ddr voltage rail (dcdc3) configuration. Set the dcdc3
DDR supply to 1.35V.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>