pinctrl: atlas7: add cs line for atlas7 nand

The nand in atlas7 has two chip select line. But in most time, the
nand only has one chip, so only one chip select line is enough.
The nand driver select this new pin group can free one chip select
line for other modules to avoid pin conflict.

Signed-off-by: Wei Chen <Wei.Chen@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>