ARM: imx/mm-imx3: conditionally compile i.MX31 and i.MX35 code

This fixes building a kernel for only one of the two SOCs. Without this
patch an i.MX31 only build fails with:

	arch/arm/mach-imx/built-in.o: In function `imx35_init_early':
	mach-bug.c:(.init.text+0x2c): undefined reference to `mxc_iomux_v3_init'
	arch/arm/mach-imx/built-in.o: In function `imx35_soc_init':
	mach-bug.c:(.init.text+0xe4): undefined reference to `mx35_revision'

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>