power: supply: axp20x_ac_power: Annotate struct axp20x_ac_power with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct axp20x_ac_power.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Chen-Yu Tsai <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Sebastian Reichel <[e-mail]>

drivers/power/supply/axp20x_ac_power.c

···
 	struct iio_channel *acin_i;
 	bool has_acin_path_sel;
 	unsigned int num_irqs;
-	unsigned int irqs[];
+	unsigned int irqs[] __counted_by(num_irqs);
 };
 
 static irqreturn_t axp20x_ac_power_irq(int irq, void *devid)