ARM: imx: soc: Switch BOARD_EARLY_INIT_F to imply on MX7

There are systems where board_early_init_f() is plain empty. Switch
the config option from "select" to "imply", to permit user to unset
the BOARD_EARLY_INIT_F if it were to be empty.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP i.MX U-Boot Team <uboot-imx@nxp.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index edc9e38..54d65f8 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -873,11 +873,11 @@
 config ARCH_MX7
 	bool "Freescale MX7"
 	select ARCH_MISC_INIT
-	select BOARD_EARLY_INIT_F
 	select CPU_V7A
 	select SYS_FSL_HAS_SEC if IMX_HAB
 	select SYS_FSL_SEC_COMPAT_4
 	select SYS_FSL_SEC_LE
+	imply BOARD_EARLY_INIT_F
 	imply MXC_GPIO
 	imply SYS_THUMB_BUILD