mfd: Define all twl-regulator feature flags in one place

twl-regulator has a collection of feature flags, some defined
in twl-core.c and  one defined in i2c/twl.h.
This is confusing for anyone adding a new feature flag.

So collect them together and place them in twl.h immediately
after the structure in which they are initially set.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>