feat(sgi): remove override for `ARM_BL31_IN_DRAM` build-option

RD-N2* variants of Neoverse reference design platforms could be
configured to boot from SRAM or DRAM. Having ARM_BL31_IN_DRAM set to 1
within the common makefile would deter these platforms from having this
flexibility. Remove the default override configuration for
`ARM_BL31_IN_DRAM`.

Signed-off-by: Rohit Mathew <rohit.mathew@arm.com>
Change-Id: I8d79969c003a984675cbe705de890b51a1f7f4ea
diff --git a/plat/arm/css/sgi/sgi-common.mk b/plat/arm/css/sgi/sgi-common.mk
index 4af579e..6c1a2dd 100644
--- a/plat/arm/css/sgi/sgi-common.mk
+++ b/plat/arm/css/sgi/sgi-common.mk
@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2018-2021, ARM Limited and Contributors. All rights reserved.
+# Copyright (c) 2018-2022, Arm Limited and Contributors. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -69,7 +69,6 @@
 
 override CSS_LOAD_SCP_IMAGES	:=	0
 override NEED_BL2U		:=	no
-override ARM_BL31_IN_DRAM	:=	1
 override ARM_PLAT_MT		:=	1
 override PSCI_EXTENDED_STATE_ID	:=	1
 override ARM_RECOM_STATE_ID_ENC	:=	1