ARM: dts: ti: drop usage of redundant compatible

Commit 229110c1 ("ARM: dts: am437x/am33xx/da850: Add new ECAP and
EPWM bindings") added ti,am3352-ehrpwm compatible which is similar to
ti,am33xx-ehrpwm but without out t,hwmod properties. But with commit
58bfbea5 ("ARM: dts: am437x/am33xx: Remove hwmod entries for ECAP
and EPWM nodes") dropped support for all ti,hwmod for ehrpwm, but
missed deprecating ti,am33xx-ehrpwm compatible. So drop ti,am33xx-ehrpwm
from DT as it is no longer needed.

ti-ehrpwn driver still support ti,am33xx-ehrpwm in order to maintain
backward compatibility.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>