secure_boot: enable chain of trust for PowerPC platforms

Chain of Trust is enabled for PowerPC platforms for Secure Boot.
CONFIG_BOARD_LATE_INIT is defined.
In board_late_init(), fsl_setenv_chain_of_trust() is called which
will perform the following:
- If boot mode is non-secure, return (No Change)
- If boot mode is secure, set the following environmet variables:
   bootdelay = 0 (To disable Boot Prompt)
   bootcmd = CONFIG_CHAIN_BOOT_CMD (Validate and execute Boot script)

Signed-off-by: Aneesh Bansal <aneesh.bansal@nxp.com>
Acked-by: Ruchika Gupta <ruchika.gupta@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>