plat/arm: sgm: Use consistent name for tb fw config node
plat/arm: sgm: Use consistent name for tb fw config node

Renamed node for trusted boot fw config from 'plat_arm_bl2' to
'tb_fw-config'.

Change-Id: I2e16b6f4d272292ec1855daafd014e851436dd9b
Signed-off-by: Manish V Badarkhe <d2342ad575f6bc1a5f5a5b89d6db0f72dd7e0e4c@arm.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware