ARM: imx: Wrap i.MX config options in MACH_IMX

Wrap all the i.MX specific options in MACH_IMX, otherwise they keep
showing up in other SoC vendor configurations. No functional change.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Tom Rini <trini@konsulko.com>