ARM: imx: enable multi-platform build

It enables multi-platform build for imx.  With ARCH_MULTI_* options
coming to play, ARCH_MXC becomes an user invisible option, while
ARCH_IMX_V4_V5 and ARCH_IMX_V6_V7 get removed.  Both imx_v4_v5_defconfig
and imx_v6_v7_defconfig get updated to adopt the changes.

AUTO_ZRELADDR and ARM_PATCH_PHYS_VIRT are selected by ARCH_MXC now to
save the duplication.

Headers timex.h and uncompress.h are not needed for multi-platform
build.  Remove them.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>