arm: dts: k3-am654: move dummy_clock to root node

The dummy_clock node is used to help the drivers probe the IO needed to
setup consoles and boot media to load firmware into the SoC.

This dummy_clock isn't a device that exists nor does it exist in the
mcu domain. So move it from cbass_mcu to the root node to avoid any
confusion.

Tested-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Bryan Brattlof <bb@ti.com>
Reviewed-by: Nishanth Menon <nm@ti.com>