ARM: OMAP: build SMP code only for OMAP4/5

The OMAP platform code assumes that SMP is only ever enabled when
CONFIG_ARCH_OMAP4 or CONFIG_SOC_OMAP5 is enabled, which is not
necessarirly true in a multiplatform configuration.

arch/arm/mach-omap2/built-in.o: In function `omap4_smp_prepare_cpus':
 :(.init.text+0x413c): undefined reference to `omap_get_wakeupgen_base'
 :(.init.text+0x415c): undefined reference to `omap_secure_apis_support'
arch/arm/mach-omap2/built-in.o: In function `omap4_boot_secondary':
 :(.cpuinit.text+0x28): undefined reference to `omap_get_wakeupgen_base'
 :(.cpuinit.text+0x3c): undefined reference to `omap_secure_apis_support'
arch/arm/mach-omap2/built-in.o: In function `omap4_cpu_die':
 :(.ref.text+0x8): undefined reference to `omap_get_wakeupgen_base'
 :(.ref.text+0x10): undefined reference to `omap_secure_apis_support'
 :(.ref.text+0x4c): undefined reference to `omap4_hotplug_cpu'
 :(.ref.text+0x50): undefined reference to `omap_secure_apis_support'

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Tony Lindgren <tony@atomide.com>