mfd: Fix twl-core sparse warning

Fixes below sparse warning.

drivers/mfd/twl-core.c:258:20: warning: symbol 'twl_map' 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>