ARM: mach-imx6q: fix spelling mistake in error message

The compatible device tree node that is searched for is imx6q-iomuxc-gpr
but was misspelled imx6q-iomux-gpr in the error handling message.

Signed-off-by: Jean Guyomarc'h <jean.guyomarch@openwide.fr>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>