ARM: kirkwood: remove automatic I2C config if DM_I2C is enabled

ARM: kirkwood: remove automatic I2C config if DM_I2C is enabled

The mach/config.h file would helpfully define CONFIG_SYS_I2C and
CONFIG_SYS_I2C_MVTWSI if CONFIG_CMD_I2C was defined by the board. This
conflicts with the way DM_I2C works. As a transitional measure don't
automatically define these if CONFIG_DM_I2C is defined. It should be
possible to remove this once all kirkwood boards are migrated to DM.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>