ARM: mvebu: allow selecting mvebu without Armada XP

Selecting only CONFIG_ARCH_MVEBU but not the respective
options for Armada 370 or Armada XP results in these
link errors:

arch/arm/mach-mvebu/built-in.o: In function `armada_xp_smp_init_cpus':
arch/arm/mach-mvebu/platsmp.c:91: undefined reference to `coherency_get_cpu_count'
arch/arm/mach-mvebu/platsmp.c:104: undefined reference to `armada_mpic_send_doorbell'
arch/arm/mach-mvebu/built-in.o: In function `armada_xp_smp_prepare_cpus':
arch/arm/mach-mvebu/platsmp.c:111: undefined reference to `set_cpu_coherent'
arch/arm/mach-mvebu/built-in.o: In function `armada_xp_boot_secondary':
arch/arm/mach-mvebu/platsmp.c:83: undefined reference to `armada_xp_boot_cpu'
arch/arm/mach-mvebu/built-in.o: In function `armada_xp_secondary_init':
arch/arm/mach-mvebu/platsmp.c:75: undefined reference to `armada_xp_mpic_smp_cpu_init'
arch/arm/mach-mvebu/built-in.o: In function `armada_xp_secondary_startup':
arch/arm/mach-mvebu/headsmp.S:46: undefined reference to `ll_set_cpu_coherent'

We can solve this by enabling all common MVEBU files that are
referenced by the SMP files. This means we enable code that
is not going to be used without a machine descriptor referencing
it, but only if the kernel is configured specifically for this
case.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Gregory Clement <gregory.clement@free-electrons.com>
Cc: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>