plat/arm/sgi: Use NT_FW_CONFIG instead of HW_CONFIG

With the two new APIs 'plat_arm_sgi_get_platform_id' and
'plat_arm_sgi_get_config_id' that are available now, BL31 need not
depend on hw_config device tree to identify the platform. In addition
to this, the existing hardware description in hw_config can be limited
to use by BL33 and not by the operating system.

So the hardware description from hw_config dts can be moved into
nt_fw_config dts and the use of hw_config dts can be removed.

Change-Id: I873b7e1e72823d3ec5d253a848e85ae724f09e49
Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>