pinctrl: lpc18xx: ensure ngroups is initialized at correct place

The initialization of ngroups is occurring at the end of the
first iteration of the outer loop, which means that the
assignment  pins[ngroups++] = i is potentially indexing into
a region outside of array pins because ngroups is not initialized.
Instead, initialize ngroups in the inner loop before the first
inner loop iteration.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>