ARM DaVinci: Changing function names for EMAC driver

DM644x is just one of a series of DaVinci chips that use the EMAC driver.
By replacing all the function names that start with dm644x_* to davinci_*
we make these function more portable. I have tested this change on my EVM.
DM6467 is another DaVinci SOC which uses the EMAC driver and i will
be sending patches that add DaVinci DM6467 support to the list soon.

Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>