mfd: twl6030: Mark of_device_id table as maybe unused

The driver references of_device_id table via of_match_device() so it
will be unused for !CONFIG_OF builds:

  drivers/mfd/twl6030-irq.c:359:34: warning: ‘twl6030_of_match’ defined but not used [-Wunused-const-variable=]
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/twl6030-irq.c

@@ -356,7 +356,7 @@ static const struct irq_domain_ops twl6030_irq_domain_ops = {
 	.xlate	= irq_domain_xlate_onetwocell,
 };
 
-static const struct of_device_id twl6030_of_match[] = {
+static const struct of_device_id twl6030_of_match[] __maybe_unused = {
 	{.compatible = "ti,twl6030", &twl6030_interrupt_mapping},
 	{.compatible = "ti,twl6032", &twl6032_interrupt_mapping},
 	{ },