mx25: move ARCH_MXC_IOMUX_V3 to the ARCH level

ARCH_MXC_IOMUX_V3 is not specific to the i.MX25 PDK platform. Thus,
ARCH_MXC_IOMUX_V3 should be selected by ARCH_MX25.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>