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>