ARM: spear: fix build error in restart.c

We can now enable mach-spear without selecting any of the
machines in a multiplatform configuration.
Doing so causes a build error that is trivial to fix by
making both the spear13xx and the spear3xx/6xx portion of
this file conditional rather than alternatives.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>