firmware: ti_sci: Add comment explaining the is_secure code

Add a comment to explain the code under is_secure condition of
ti_sci_do_xfer. This will help avoid confusion amongst people who may in
future touch upon this code.

Reviewed-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Dhruva Gole <d-gole@ti.com>