Platform/ARM/Sgi: Adapt to changes in system-id DT node.

The 'system-id' node of HW_CONFIG device tree has been updated to have
a new property 'config-id' to hold the platform configuration value.
To adapt to this change, SGI_PLATFORM_DESCRIPTOR gets a new member
named 'ConfigId' and the function GetSgiPlatformId has been renamed
to GetSgiSystemId which takes the SGI_PLATFORM_DESCRIPTOR HoB as the
function parameter.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>