net: ethernet: ti: cpsw: drop TI_DAVINCI_CPDMA config option

Both drivers CPSW and EMAC can't work without CPDMA, hence simplify build
of those drivers by always linking davinci_cpdma and drop TI_DAVINCI_CPDMA
config option.
Note. the davinci_emac driver module was changed to "ti_davinci_emac" to
make build work.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>