plat: bcm_vk: core specific init

Add initilizations for:
- Cortex-A72 L2 Controller configurations. This initialization to
 be done when cluster is in quiscent state.
- 'ICC_SRE_EL3' init to allow GIC V3 ICC_SRE_ELx system interface.
 This initialization can be done at 'EL3' only.

Signed-off-by: Sandeep Tripathy <sandeep.tripathy@broadcom.com>