power: bq27xxx_battery: move irq handler to i2c section

The IRQ handler is not used by the platform based
code resulting in a 'defined but not used' warning,
if CONFIG_BQ27XXX_I2C is not enabled.

Signed-off-by: Sebastian Reichel <sre@kernel.org>
Acked-by: Pali Rohár <pali.rohar@gmail.com>
Acked-by: Andrew F. Davis <afd@ti.com>