power: pmic: tps65218: Add DCDC3 configuration

Some boards like am437x-gp-evm require dcdc3 also to be configured
as it feeds on to ddr. Hence add the capability as well.

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