feat(partition): add support to use backup GPT header

Currently we just use primary GPT header which is located in second
entry after MBR header, but if this block is corrupted or CRC
mismatch occurs we could try to use the backup GPT header located at
LBAn and GPT entries following this from LBA-33.

Add suitable warning messages before returning any errors to identify
the cause of issue.

Change-Id: I0018ae9eafbacb683a18784d2c8bd917c70f50e1
Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>