Add Kconfig options for USB keyboard and use them for cm-fx6.

Cc: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: NNikita Kiryanov <nikita@compulab.co.il>
Acked-by: NIgor Grinberg <grinberg@compulab.co.il>