ARM: mvebu: fix build without platforms selected

ARM: mvebu: fix build without platforms selected

When building a multiplatform kernel that enables 'ARCH_MVEBU' but
none of the individual options under it, we get this link error:

arch/arm/mach-mvebu/built-in.o: In function `mvebu_armada375_smp_wa_init':
:(.text+0x190): undefined reference to `mvebu_setup_boot_addr_wa'

The best solution seems to be to ensure that in this configuration,
we don't actually build any of the mvebu code.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lkml.kernel.org/r/7339332.ZE2mWIdyDh@wuerfel
Signed-off-by: Jason Cooper <jason@lakedaemon.net>