arm64: dts: ti: Add PWM overlay for AM68 SK

The AM68 SK has PWM output pins connected to its RPi headers on
J3 Connector. This overlay will set the appropriate pinmux and
enable PWM on those pins.

Signed-off-by: Sinthu Raja <sinthu.raja@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>