ARM: dts: keystone*: Standardize license with SPDX tag

Standardize the licenses with SPDX license tag and standardize TI's
copyright statement to be consistently after file description.

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