fix compilation of i.MX31 platforms

mxc_iomux_v3_init() is defined in arch/arm/plat-mxc/iomux-v3.c, which is
not linked for i.MX31 and produces an undefined reference error. Fix this
by building the offending code only for i.MX35.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>