bcm63xx: pinctrl: fix BCM6348 groups

bcm63xx: pinctrl: fix BCM6348 groups

The current driver has some troubles:
 - Some groupings are wrong.
 - The pinctrl group0 owns pins never used (at least in Openwrt) for any
   pinmux. The driver hijacks all the pins on the group avoiding any other
   use, spite they're free. I.e. for buttons, causing this kernel error:
   [  4.735928] gpio-keys-polled keys: unable to claim gpio 479, err=-22
   [  4.742642] gpio-keys-polled: probe of keys failed with error -22
 - Minor errors about groupings on the documentation
 - Missing "diag" grouping in dtsi
 - Wrong groupings in dtsi

Fix it by setting the correct groups.
And relax the pin capturing, letting the gpios belonging to any group to
be used for other purposes like buttons. This was the behavior with stock
firmwares and old OpenWrt versions which never caused any trouble.

Signed-off-by: Daniel González Cabanelas <dgcbueu@gmail.com>
(Cherry-picked from commit 50cb3a750f)