armada38x: implement gateclk clkdev methods

Implement basic clkdev methods required by the generic clk_gate device.
This should make it functional, and prevent panics when invoking the
clock's CLKNODE_GET_GATE method (e.g. with sysctl -a).

Additionally, we need to fill in other key fields of the clk_gate_def
structure before registering each clk.

PR: 278188
Reported by: solo_code@protonmail.com
Tested by: solo_code@protonmail.com
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48419