ARM: am3517_evm: Disable DM_I2C_COMPAT

DM_I2C_COMPAT is somehow being enabled outside of Kconfig, so
this explicitly undefines it in the header file, and brackets
the I2C initialization around an #ifdef to not manually
initialize the I2C controller when the DM_I2C is enabled.

Signed-off-by: Adam Ford <aford173@gmail.com>
Tested-by: Derald D. Woods <woods.technical@gmail.com>