pwm: tiehrpwm: Enable compilation for ARCH_K3

K3 devices have the same EHRPWM IP as OMAP SoCs. Enable the 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 Raghavendra <vigneshr@ti.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>