dt-bindings: clock: Add binding documentation for TI EHRPWM TBCLK

Add DT bindings for TI EHRPWM's TimeBase clock (TBCLK) on TI's AM654 SoC.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Link: https://lkml.kernel.org/r/20200227053529.16479-2-vigneshr@ti.com


Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>