dt-bindings: ti-sysc: Add SPDX license identifier

Add the appropriate SPDX license identifier to the common
TI sysc bindings header file.

Signed-off-by: Suman Anna <s-anna@ti.com>
Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>