The ti,clksel binding puts the register offset on the parent node; a
child's "reg" is only an index matching its unit address. The mux,
divider and gate clock drivers read the child's "reg" first and use it
as the offset, which on am335x makes unaligned syscon accesses and
faults while registering the mux clocks:

ti_clksel0: <TI CLKSEL> on ofw_clkbus0
ti_mux12: <TI Mux Clock> on ti_clksel0
Fatal kernel mode data abort: 'Alignment Fault' on read
FSR=00000001, FAR=ffd00001

Take the offset from the parent when it is a ti,clksel node, otherwise
from the child's "reg", and fail to attach if neither exists rather than
use a wrong offset.

Boots to multi-user on a BeagleBone Black Rev B3 with the full stack
applied, rebased onto stable/15. Without this change the boot dies at the
alignment fault above while registering mux clocks; with it, the TI clock
tree registers completely (mux, gate, divider and DPLL nodes all attach)
and the board reaches login with zero device attach failures across six
reboot cycles.

Full evidence: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=297800