ARM: dts: AM43x: Add TPS65218 device tree nodes

Add TPS65218 device tree nodes. i2c clock frequency setting
also added as part of tps65218 nodes addition. As i2c clock
enabling is required.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>