Add Device Tree binding document for the generic PWM based tachometer. Bug 3621819 Change-Id: Ic8184e3e86d8708fd776d276e4d9994e4103b5ed Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com> Reviewed-on: https://git-master.nvidia.com/r/c/linux-nv-oot/+/2703730 GVS: Gerrit_Virtual_Submit