twl4030-core: move to drivers/mfd

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Move twl4030-core to drivers/mfd from drivers/i2c/chips,
along with its Kconfig and Makefile help.

The intent here is:  no functional change, match the patch
sent to LKML this morning (except, fix that spelling error
in the new Kconfig text).

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>