nxp: added the makefile helper macros

NXP specifc macro SET_NXP_MAKE_FLAG is added.

NXP has pool of multiple IPs. This macro helps:
- In soc.mk, this macro help the selected IP source files to be included
  for that SoC.
  -- The set of IPs required for one NXP SoC is different to the set of IPs
     required by another NXP SoC.

- For the same SoC,
  -- For one feature, the IP may be required in both BL2 and BL31.
  -- Without the above feature, that IP may be required in one.
     This macro help in selecting the inclusion of source and header files to:
     --- BL2 only
     --- BL31 only
     --- COMM (used by BL2 and BL31)
Signed-off-by: Pankaj Gupta <pankaj.gupta@nxp.com>
Change-Id: I2cdb13b89aa815fc5219cf8bfb9666d0a9f78765

plat/nxp/common/plat_make_helper/plat_build_macros.mk

+#
+# Copyright (c) 2020, NXP.
+#
+# SPDX-License-Identifier: BSD-3-Clause
+#
+#
+
+define	SET_NXP_MAKE_FLAG
+$1	:= yes
+$2_$1	:= yes
+endef