ARM: nomadik: hide MACH_NOMADIK_8815NHK in Kconfig

The nomadik multiplatform support made it possible to select
MACH_NOMADIK_8815NHK without selecting ARCH_NOMADIK, which leads
to build errors when we also select ARMv6/v7 targets. Adding the
ifdef here restores the intended behavior.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>