SECURE BOOT: separate functions for reading keys

Separate functions are created for reading and checking the
sanity of Public keys:
- read_validate_single_key
- read_validate_ie_tbl
- read_validate_srk_table

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