As this is only useful when booting with ATAGs, which are now largely
disabled, remove this value for the remaining platforms.  We have a few
places in the code that had been testing for MACH_TYPE as a sort of
internal logic.  Update those to use different but still correct CONFIG
symbols.

Cc: Phil Sutter <phil@nwl.cc>
Signed-off-by: Tom Rini <trini@konsulko.com>