A number of pins on the imx233 are standard and manually calling
functions to acquire, set function/drive/output is painful. This
will become unmanageable when we will add support for the other
stmp chips.
Introduce the concept of virtual pin which is a way to completely
describe a virtual pin (virtual because pins are muxed).
Change-Id: I01b6e040945648e58e1d1abab06529c9571c5f10
