mfd: Fix twl_probe section mismatch warning in mfd/twl-core.c

Fix the following section mismatch warning when building
omap2plus_defconfig:

WARNING: vmlinux.o(.data+0x47d7c): Section mismatch in reference
from the variable twl_driver to the function .init.text:twl_probe()

Signed-off-by: Bryan Wu <bryan.wu@canonical.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>