mfd: Update twl6040 Kconfig to avoid build breakage

twl6040 needs CONFIG_IRQ_DOMAIN to compile, without this we have:
drivers/mfd/twl6040-irq.c: In function 'twl6040_irq_init':
drivers/mfd/twl6040-irq.c:164:2: error: implicit declaration of function 'irq_domain_add_legacy'
drivers/mfd/twl6040-irq.c:165:11: error: 'irq_domain_simple_ops' undeclared (first use in this function)
drivers/mfd/twl6040-irq.c:165:11: note: each undeclared identifier is reported only once for each function it appears in

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>