pinctrl: lpc18xx: mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Addresses-Coverity-ID: 1292308 ("Missing break in switch")
Addresses-Coverity-ID: 1292309 ("Missing break in switch")
Addresses-Coverity-ID: 1309546 ("Missing break in switch")
Addresses-Coverity-ID: 1357369 ("Missing break in switch")
Addresses-Coverity-ID: 1357389 ("Missing break in switch")
Reviewed-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>