warp7: Print secure/non-secure mode info

warp7 has two targets:

- warp7_defconfig: boots in non-secure mode
- warp7_secure_defconfig: boots in secure mode

Print the mode that is being used to help users to easily identify
which target is running on the board.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>