ARM: trusted_foundations: fix compile error on non-SMP

The setup_max_cpus variable is only defined if CONFIG_SMP is set. Add
a preprocessor condition to avoid the following compilation error if
CONFIG_SMP is not set:

    arch/arm/include/asm/trusted_foundations.h: In function 'register_trusted_foundations':
    arch/arm/include/asm/trusted_foundations.h:57:2: error: 'setup_max_cpus' undeclared (first use in this function)

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reported-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>