Add group configuration for uarts that are cut down
variants, the standard being full, i.e. all signals,
flow control, i.e. rx/tx and cts/rts, and rx/tx only.

This allows us to be more precise in which pins we're
actually using.
Signed-off-by: Jesper Nilsson <jesper.nilsson@axis.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>