Remove clock-frequency properties from Armada 38x timer nodes

Since the timers' base frequency setting is added to the platform code,
this patch removes clock-frequency properties from global
and twd timers, aligning both to the Linux device tree.

Submitted by: Patryk Duda <pdk@semihalf.com>
Reviewed by: cognet (mentor)
Approved by: cognet (mentor)
Obtained from: Semihalf
Differential Revision: https://reviews.freebsd.org/D11882