Move Ti AM335x to dev/extres/clk framework.

Re-implement clocks for these SoC by using now standard extres/clk framework.
This is necessary for future expansion of these. The new implementation
is (due to the size of the patch) only the initial (minimum) version.
It will be updated/expanded with a subsequent set of particular patches.

This patch is also not tested on OMAP4 based boards (BeagleBone),
so all possible issues should be (and will be) fixed by ASAP once
identified.

Submited by: Oskar Holmlund (oskar.holmlund@ohdata.se)
Differential Revision: https://reviews.freebsd.org/D25118