pinctrl: imx: attach iomuxc device to gpr syscon

Commit bdb0066d ("mfd: syscon: Decouple syscon interface from platform
devices") added the possibility to register syscon devices without
associated platform device. This also removed regmap debugfs facilities,
which don't work without a device. This patch associates the syscon regmap
that handles the IOMUX controller's general purpose registers with the
pinctrl device so that the GPR registers appear in the regmap debugfs
directory again. For example, on i.MX6Q the GPR registers now can be
read from /sys/kernel/debug/regmap/20e0000.iomuxc-gpr/registers.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>