plat/arm/sgi: add system-id node in HW_CONFIG dts

Dynamically populating the 'system-id' node in the HW_CONFIG dts makes
it difficult to enforce memory overlap checks. So add the system-id node
in the HW_CONFIG dts file as a place holder with 'platform-id' and
'config-id' set to zero.

The code at BL2 stage determines the values of 'platform-id' and
'config-id' at runtime and updates the corresponding fields in the
system-id node of HW_CONFIG dts.

Change-Id: I2ca9980b994ac418da8afa0c72716ede10aff68a
Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>