pinctrl: artpec6: dt: add smaller groups for uarts

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.

Unfortunately the existing naming scheme leaves things
to be desired, e.g. uart3grp0 means RX/TX and CTS/RTS,
yet uart0grp0 means all pins.
Since the exising suffixes have different meaning for
different uarts, and the fact that we cannot change
the name of existing groups, makes it hard to use a
descriptive name for the newly added groups.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>