Changes GPIO session interface to a one-GPIO-pin-per-session style. Moreover,
this commit introduces a generic driver interface for GPIO drivers. Thereby
generalizes root- and session component for GPIO.
There is no obvious reason for having two different SPEC variables, definitions,
and pathes for the Pandaboard platform. It even lead to problems regarding the
omap4 framebuffer driver (look at issue #505 and #506).
