mfd: axp20x: Skip of_device_id table when !CONFIG_OF

The driver can match either via ACPI or OF.  Its of_device_id table is
referenced via of_match_ptr() so it will be unused for !CONFIG_OF
builds:

  drivers/mfd/axp20x-i2c.c:60:34: warning: ‘axp20x_i2c_of_match’ defined but not used [-Wunused-const-variable=]

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>