MACH_... is reserved for machine support, so use SOC as prefix, not MACH.

This introduces new symbols SOC_IMX1, SOC_IMX21 and SOC_IMX27.  They are
selected by the old symbols for now.  There is no substitute for MACH_MX2
as most usages of MX2 only means MX21 + MX27 but not MX25.

Later the choice about CPU and CPU family should go away and the individual
machines should select the right SOC symbol.  This is a precondition to
support more than one SOC in a single kernel image.
Signed-off-by: NUwe Kleine-König <u.kleine-koenig@pengutronix.de>