arm: Remove spear600 boards and the rest of SPEAr support

These boards have not been converted to CONFIG_DM_USB by the deadline
and is also missing conversion to CONFIG_DM.  Remove them.  As this is
the last of the SPEAr platforms, so remove the rest of the remaining
support as well.

Cc: Vipin Kumar <vipin.kumar@st.com>
Signed-off-by: Tom Rini <trini@konsulko.com>