plat/arm/sgi: add chip_id and multi_chip_mode to platform variant info

plat/arm/sgi: add chip_id and multi_chip_mode to platform variant info

Multi-chip platforms have two or more identical chips connected using a
high speed coherent link. In order to identify such platforms,
add chip_id and multi_chip_mode information in the platform variant
info structure. The values of these two new elements is populated
during boot.

Change-Id: Ie6e89cb33b3f0f408814f6239cd06647053e23ed
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>