pinctrl: imx: do not implicitly set pin regs to -1

Commit 3dac1918 ("pinctrl: imx: detect uninitialized pins") needs
the values in struct imx_pin_reg to be -1. This has been done in a
rather unorthodox way by setting the memory to 0xff using memset...
Use a proper for loop to initialize the whole array with -1.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>