Platform/ARM/Sgi: Use NT_FW_CONFIG instead of HW_CONFIG

The hardware configuration in HW_CONFIG dts is not being
passed onto the operating system but used and terminated
at edk2 boot stage (BL33). So, as per the recommendations
of the trusted-firmware design, the hardware description
specified in NT_FW_CONFIG dts should be used instead of
HW_CONFIG dts.

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