MX28: use a clear name for DDR2 initialization

The mx28 prefix has been added to the initialization data and function
so it is clear by which SoC it is used as i.MX233 will have a specific
one. While on that, we also change it to static.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Acked-by: Marek Vasut <marex@denx.de>