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 <[e-mail]>
Reported-by: Russell King <[e-mail]>
Acked-by: Stephen Warren <[e-mail]>
Signed-off-by: Arnd Bergmann <[e-mail]>

arch/arm/include/asm/trusted_foundations.h

···
 	 */
 	pr_err("No support for Trusted Foundations, continuing in degraded mode.\n");
 	pr_err("Secondary processors as well as CPU PM will be disabled.\n");
+#if IS_ENABLED(CONFIG_SMP)
 	setup_max_cpus = 0;
+#endif
 	cpu_idle_poll_ctrl(true);
 }