DaVinci: move AEMIF #define's to the proper headers

Currently each DaVinci board file #define's its own version of the EMIFA base
addresses (all named DAVINCI_ASYNC_EMIF_*_BASE), which leads to duplication.
Move these #define's to the SoC specific headers, changing their prefixes from
'DAVINCI' to the 'DM355', 'DM644X', and 'DM646X' since all these base addresses
are SoC specific...

And while at it, rename DM646X_ASYNC_EMIF_DATA_CE0_BASE to
DM646X_ASYNC_EMIF_CS2_SPACE_BASE in order to match the DM646x datasheet.

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>