pinctrl: lpc18xx: create pin cap lookup helper

Both pconf_get_pin and pconf_set_pin needs to lookup pin cap based
on the pin number. Create a common helper function that both
functions can use that also handles the case where no pin number is
found in the pins array.

This also fixes a small bug in pconf_get_pin where pconf_get_i2c0
would use the pins array index rather than the pin number.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>