dt-bindings: pwm: pwm-tiecap: Add compatible string for AM64 SoC

Add compatible string for AM64 SoC in device tree binding.
IP is compatible with ti,am3352-ecap, so adding the AM64 compatible
under enum of one of the compatible list entry.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>