ARM: MX3: modularize 'mx31lite' code

This commit splits the support code for LogicPD's mx31lite hardware
into module and board specific parts.

This introduces a new mandatory coreparam called 'mx31lite_baseboard'
which specifies the base board support to use. For now, only the LiteKit
development board is supported, and developers of own boards are
encouraged to use that as reference.

The UART support moved to the board code.
Some comments were amended along the way.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>