arm: mach-k3: j72xx: add new 'virtual' mux

In order for the Cortex-A72s to operate at different frequencies other
than the default 2GHz, add in a new 'virtual' mux (a mux that does not
physically exist in the clock tree) that can be selected.

CC: Vishal Mahaveer <vishalm@ti.com>
Signed-off-by: Bryan Brattlof <bb@ti.com>
Signed-off-by: Apurva Nandan <a-nandan@ti.com>
Signed-off-by: Reid Tonking <reidt@ti.com>
Reviewed-by: Udit Kumar <u-kumar1@ti.com>