mfd: Fix twl6040 build failure

Without it we get:

 CC      drivers/mfd/twl6040-core.o
drivers/mfd/twl6040-core.c: In function ‘twl6040_has_vibra’:
drivers/mfd/twl6040-core.c:55:2: error: implicit declaration of function
‘of_find_node_by_name’ [-Werror=implicit-function-declaration]

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>