ARM: dts: imx6sl: add keypad support for i.mx6sl-evk board.

i.MX6SL EVK board has a 3*3 keypad matrix to support 8 keypads,
enable them, the keymap is as below:

SW6:  MATRIX_KEY(0x0, 0x0, KEY_UP)         /* ROW0, COL0 */
SW7:  MATRIX_KEY(0x0, 0x1, KEY_DOWN)       /* ROW0, COL1 */
SW8:  MATRIX_KEY(0x0, 0x2, KEY_ENTER)      /* ROW0, COL2 */
SW9:  MATRIX_KEY(0x1, 0x0, KEY_HOME)       /* ROW1, COL0 */
SW10: MATRIX_KEY(0x1, 0x1, KEY_RIGHT)      /* ROW1, COL1 */
SW11: MATRIX_KEY(0x1, 0x2, KEY_LEFT)       /* ROW1, COL2 */
SW12: MATRIX_KEY(0x2, 0x0, KEY_VOLUMEDOWN) /* ROW2, COL0 */
SW13: MATRIX_KEY(0x2, 0x1, KEY_VOLUMEUP)   /* ROW2, COL1 */

Signed-off-by: Anson Huang <b20788@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>