firmware: ti_sci: Switch to SPDX Licensing

Switch to SPDX licensing and drop the GPL text which comes redundant.

Acked-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>