feat(corstone1000): identify bank to load fip

Secure enclave decides the boot bank based on the firmware update
state of the system and updates the boot bank information at a given
location in the flash. In this commit, bl2 reads the given flash
location to indentify the bank from which it should load fip from.

Signed-off-by: Satish Kumar <satish.kumar01@arm.com>
Signed-off-by: Vishnu Banavath <vishnu.banavath@arm.com>
Change-Id: I7f0f4ffc97189c9deb99db44afcd966082ffbf21