ARM: socfpga: remove redundant config.mk

Because all the SOCFPGA boards define CONFIG_SPL_FRAMEWORK
(see include/configs/socfpga_common.h), u-boot.img is automatically
added to the target image list by the top Makefile.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

arch/arm/cpu/armv7/socfpga/config.mk

@@ -1,8 +0,0 @@
-#
-# Copyright (C) 2011, Texas Instruments, Incorporated - http://www.ti.com/
-#
-# SPDX-License-Identifier:	GPL-2.0+
-#
-ifndef CONFIG_SPL_BUILD
-ALL-y	+= u-boot.img
-endif