mfd: provide and use setup hook for tc6393xb

mfd: provide and use setup hook for tc6393xb

Instead of using bitfields for initial gpio setup,
provide generic setup/teardown hooks that can be used
to set the gpio states, register child devices, etc.

Signed-off-by: Dmitry Baryshkov <dbaryshkov@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@openedhand.com>