Fix a number of small issues visible when GPIO is enabled:

- Correct missing default for !ETRAXFS in Kconfig
- Remove information on number of bits for some Kconfigs
  related to the GPIO, they are different in ETRAX FS and ARTPEC-3
- Fix compile warning in ARTPEC-3 GPIO driver
Signed-off-by: NJesper Nilsson <jesper.nilsson@axis.com>