ARM: spear6xx: remove board selection options

There are no board specific configurations that need user
intervention, so just make MACH_SPEAR600 the silent default
for ARCH_SPEAR6XX to prevent users from turning it off, which
would result in a build error.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reported-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: Viresh Kumar <viresh.kumar@st.com>