pinctrl: move group lookup to core

Now also the core needs to look up pin groups so move the lookup
function there and expose it in the internal header.

Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

diff --cc drivers/pinctrl/core.c
Simple merge

diff --cc drivers/pinctrl/core.h
Simple merge

diff --cc drivers/pinctrl/pinmux.c
Simple merge