arm: exynos: arndale: Remove unused CONFIG_DM_I2C_COMPAT

The CONFIG_DM_I2C_COMPAT was introduced in
include/configs/exynos5-common.h in commit 189d8016 ("exynos5:
enable dm i2c") and then it propagated up to configs/arndale_defconfig.
However since beginning the Arndale board (Exynos5250) was not using
I2C.

In fact, the Arndale board is not configuring its PMIC (S5M8767) which
uses I2C bus.  This setting can be thus safely removed to fix build
warning:

    This board uses CONFIG_DM_I2C_COMPAT. Please remove
    (possibly in a subsequent patch in your series)
    before sending patches to the mailing list.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>