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 <Manish.Badarkhe@arm.com>

plat/arm/css/sgm/fdts/sgm_tb_fw_config.dts

@@ -8,7 +8,7 @@
 
 / {
 	/* Platform Config */
-	plat_arm_bl2 {
+	tb_fw-config {
 		compatible = "arm,tb_fw";
 		hw_config_addr = <0x0 0x83000000>;
 		hw_config_max_size = <0x01000000>;