MX51EVK: UART does not print out the early information

The early bootup information is not print out due to
the UART pin iomux not set up correctly before board_init

Add the board_early_init_f function and enable the
CONFIG_BOARD_EARLY_INIT_F. Move the UART pin setting
from board_init to board_early_init_f function.

This patch also move the FEC pin iomux setup to the
board_early_init_f.

Signed-off-by: Jason Liu <r64343@freescale.com>