Add compatible string for AM64 SoC in device tree binding.
EPWM IP in AM64 does not support High resolution, so named epwm instead
of ehrpwm in compatible. However IP is till compatible with features
supported by driver with ti,am3352-ehrpwm compatible.

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