boards: frdm_k64f: add SD/MMC support

The FRDM-K64F board has a SD/MMC slot. This patch modifies the DTS
files so that this feature works out of the box. This includes
configuring the SPI peripheral connected to the slot on the board.
This change follows the SD/MMC support for other boards, e.g.
Olimexino-STM32. It was tested on a FRDM-K64F board with the
fat_fs sample.

Signed-off-by: Marek Materzok <tilk@tilk.eu>