ARM: SPEAr: conditionalize SMP code

Some constant definitions are only defined for spear13xx, so
we must not attempt to build SPEAr SMP support when that
SoC is not enabled.

arch/arm/mach-spear/platsmp.c:25:35:
 error: 'VA_SCU_BASE' undeclared here (not in a function)
 arch/arm/mach-spear/platsmp.c: In function 'spear13xx_smp_prepare_cpus':
 arch/arm/mach-spear/platsmp.c:111:58: error: 'SYS_LOCATION' undeclared (first use in this function)

Cc: Viresh Kumar <viresh.linux@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>