The `board/freescale/twr-k70f120m/board.c` file was copied from
`board/freescale/twr-k60n512/board.c` without code changes, because the
Ethernet pin configuration is compatible on these two boards.

`cpu/arm_cortexm3/kinetis/clock.c` was updated to support the MCG
(Multipurpose Clock Generator) internal structure on K70 @ 120 MHz
which is different from the MCG on K60 @ 100 MHz.

The U-Boot configuration file (include/configs/twr-k70f120m.h) was
copied from the corresponding file for the TWR-K60N512 board and
customized for the TWR-K70F120M board:
    1. UART pins are different
    2. There is external RAM on the TWR-K70F120M board
    3. The in-MCU flash is 1 MB in size
    4. The clock configuration was updated (120 MHz core clock)
    5. The K70 MCU has 6 GPIO ports (instead of 5 ports on K60)