fbdev: mxsfb: Add support for mx6sl and mx6sx

mx6sl and mx6sx share the same LCD controller as mx23 and mx28.

Add support for it.

The basic difference is the number of clocks that are required:

- mx23/mx28: only one clock
- mx6sl: two clocks
- mx6sx: three clocks

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>