Add initial support for mx6slevk board

mx6slevk board is a development board from Freescale based on the mx6 solo lite
processor.

For details about mx6slevk, please refer to:
http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=IMX6SLEVK&parentCode=i.MX6SL&fpsp=1

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>