ARM: SPEAr: conditionalize l2x0 support

If the cache controller implementation is disabled at build time,
we must not call any functions related to it.

arch/arm/mach-spear/built-in.o: In function `spear13xx_l2x0_init':
arch/arm/mach-spear/spear13xx.c:47: undefined reference to `l2x0_init'

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