pinctrl: imx: detect uninitialized pins

The pinctrl driver initialized the register offsets for the pins
with 0. On Vybrid an offset of 0 is a valid offset for the pinctrl
mux register. So far, this was solved using the ZERO_OFFSET_VALID
flag which allowed offsets of 0. However, this does not allow to
verify whether a pins struct imx_pmx_func was initialized or not.

Use signed offset values for register offsets and initialize those
with -1 in order to detect uninitialized offset values reliable.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>