gpio: remove some legacy comments in build files

The build files for drivers/gpio has some wording and comments
about the directory being reserved exclusively for GPIO expanders
(according to the gpio.txt file these are on external busses) and
this has been false for some time. We already have PL061 and
Xilinx drivers which are in silicon and now I'm moving more
silicon drivers here, so delete this and reword it a bit.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>