firmware: ti_sci: allow frequency change for disabled clocks by default

If a clock is disabled, its frequency should be allowed to change as
it is no longer in use. Add a flag towards this to the firmware clock
API handler routines.

Acked-by: Nishanth Menon <nm@ti.com>
Tested-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>