riscv/mmiowb: Hook up mmwiob() implementation to asm-generic code

In a bid to kill off explicit mmiowb() usage in driver code, hook up
the asm-generic mmiowb() tracking code for riscv, so that an mmiowb()
is automatically issued from spin_unlock() if an I/O write was performed
in the critical section.

Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index eb56c82..6e30e81 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -48,6 +48,7 @@
 	select RISCV_TIMER
 	select GENERIC_IRQ_MULTI_HANDLER
 	select ARCH_HAS_PTE_SPECIAL
+	select ARCH_HAS_MMIOWB
 	select HAVE_EBPF_JIT if 64BIT
 
 config MMU