imx: configs: Cleanup CONFIG_SECURE_BOOT comments

Since commit 6e1f4d2652e7 ("arm: imx-common: add SECURE_BOOT option
to Kconfig") the SECURE_BOOT option is selected through Kconfig.

Cleanup comments in code to align with this change.

Signed-off-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
2 files changed