feat(imx8m): enable conditional build for SDEI

SDEI support on imx8m is an optional feature, so
make it conditional build, not enabled by default.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Change-Id: I6e7e8d77959ea352bc019f8468793992ec7ecfc4
diff --git a/plat/imx/imx8m/imx8mp/platform.mk b/plat/imx/imx8m/imx8mp/platform.mk
index 823b5d6..d82ed4f 100644
--- a/plat/imx/imx8m/imx8mp/platform.mk
+++ b/plat/imx/imx8m/imx8mp/platform.mk
@@ -1,5 +1,5 @@
 #
-# Copyright 2019-2020 NXP
+# Copyright 2019-2022 NXP
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -30,8 +30,6 @@
 				plat/imx/imx8m/imx8mp/imx8mp_psci.c		\
 				plat/imx/imx8m/imx8mp/gpc.c			\
 				plat/imx/common/imx8_topology.c			\
-				plat/imx/common/imx_ehf.c                       \
-				plat/imx/common/imx_sdei.c                      \
 				plat/imx/common/imx_sip_handler.c		\
 				plat/imx/common/imx_sip_svc.c			\
 				plat/imx/common/imx_uart_console.S		\
@@ -149,5 +147,8 @@
 IMX_BOOT_UART_BASE	?=	0x30890000
 $(eval $(call add_define,IMX_BOOT_UART_BASE))
 
-EL3_EXCEPTION_HANDLING := 1
-SDEI_SUPPORT := 1
+EL3_EXCEPTION_HANDLING := $(SDEI_SUPPORT)
+ifeq (${SDEI_SUPPORT}, 1)
+BL31_SOURCES 		+= 	plat/imx/common/imx_ehf.c	\
+				plat/imx/common/imx_sdei.c
+endif