clk: keystone: sci-clk: probe clocks from DT instead of firmware

Probing all the available clocks from the PM firmware takes quite a lot
of time, increasing boot time. Instead, implement functionality that
parses only the used clocks from DT, and registers these to clock core.
This way, the boot time is greatly improved.

Additionally, provide a Kconfig option for parsing all the clocks from
firmware, if someone requires this. It is mostly useful as a debugging
functionality if we want to inspect the whole clock tree.

Acked-by: Santosh Shilimkar <ssantosh@kernel.org>
Signed-off-by: Tero Kristo <t-kristo@ti.com>