drivers/video: use strings to specify the Freescale DIU monitor port

Instead of using ill-defined numbers (0, 1, and 2) for the monitor port, allow
the user to specify the port by name ("dvi", "lvds", or "dlvds").  This works
on the kernel command line, the module command-line, and the sysfs "monitor"
device.

Note that changing the monitor port does not currently work on the P1022DS,
because the code that talks to the PIXIS FPGA is broken.

Signed-off-by: Timur Tabi <timur@freescale.com>
Acked-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>