arch: Use asm-generic header for asm/mmiowb.h

Hook up asm-generic/mmiowb.h to Kbuild for all architectures so that we
can subsequently include asm/mmiowb.h from core code.

Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/alpha/include/asm/Kbuild

@@ -9,6 +9,7 @@ generic-y += irq_work.h
 generic-y += kvm_para.h
 generic-y += mcs_spinlock.h
 generic-y += mm-arch-hooks.h
+generic-y += mmiowb.h
 generic-y += preempt.h
 generic-y += sections.h
 generic-y += trace_clock.h