mach-ux500: dynamic UIB (user interface boards) detection

Add support for dynamic detection of the UIB used (at the cost of one i2c error
on the lesser-used UIB) and also provide an override via a command line
parameter if needed.

Signed-off-by: Rabin Vincent <rabin.vincent@stericsson.com>
Signed-off-by: Sundar Iyer <sundar.iyer@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>