mfd: Fix twl-irq function declaration warnings

Fixes following sparse warnings for twl4030 and twl6030 irq files.

drivers/mfd/twl4030-irq.c:783:5: warning: symbol 'twl4030_init_irq' was not
declared. Should it be static?
drivers/mfd/twl4030-irq.c:863:5: warning: symbol 'twl4030_exit_irq' was not
declared. Should it be static?
drivers/mfd/twl4030-irq.c:873:5: warning: symbol 'twl4030_init_chip_irq' was
not declared. Should it be static?

drivers/mfd/twl6030-irq.c:226:5: warning: symbol 'twl6030_init_irq' was not
declared. Should it be static?
drivers/mfd/twl6030-irq.c:290:5: warning: symbol 'twl6030_exit_irq' was not
declared. Should it be static?

Signed-off-by: G, Manjunath Kondaiah <manjugk@ti.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: Tony Lindgren <tony@atomide.com>
Cc: Nishanth Menon <nm@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>