ARM: da850-evm: Enable DM_I2C

With DM now enabled with the device tree pulled from Linux, we can
enable DM_I2C in U-Boot.

Reviewed-by: Jagan Teki <jagan@openedev.com>
Signed-off-by: Adam Ford <aford173@gmail.com>
[trini: Add DM_I2C_COMPAT to da850_am18xxevm to fix warning]
Signed-off-by: Tom Rini <trini@konsulko.com>