pwm: Enable TI ECAP driver for ARCH_K3

K3 devices have the same ECAP IP as OMAP SoCs. Enable driver to be built
for K3 devices. Also, drop reference to AM33xx in help text, as IP is
found on multiple TI SoCs.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>