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>