arm: ti am335x ehrpwm remove sysctl interface

To minimize the maintenance time of this driver when new features
are added the legacy sysctl interface has to go.

Approved by: manu (mentor)
Reviewed by: Dr. Rolf Jansen (freebsd-rj_obsigna.com)
Differential revision: https://reviews.freebsd.org/D29546