Since ACPI_PTR() is used to NULLify the value when !CONFIG_ACPI,
struct axp20x_i2c_acpi_match becomes defined by unused.

This squashes the current W=1 kernel builds warning:

 drivers/mfd/axp20x-i2c.c:82:36: warning: ‘axp20x_i2c_acpi_match’ defined but not used [-Wunused-const-variable=]

Cc: Chen-Yu Tsai <wens@csie.org>
Cc: Carlo Caione <carlo@caione.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>