ARM: imx: merge plat-mxc into mach-imx

It's really unnecessary to have plat-mxc, and let's merge it into
mach-imx.  It's pretty much just a bunch of file renaming and
Kconfig/Makefile merge.

To make the change less invasive, we keep using Kconfig symbol
CONFIG_ARCH_MXC for mach-imx sub-architecture.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>