arch: hexagon: include: asm: add generic macro 'mmiowb' in "io.h"

Need dumy mmiowb(), or can not pass compiling, the related error with
allmodconfig:

    CC [M]  drivers/mmc/host/sdhci.o
  drivers/mmc/host/sdhci.c: In function 'sdhci_request':
  drivers/mmc/host/sdhci.c:1409:2: error: implicit declaration of function 'mmiowb' [-Werror=implicit-function-declaration]
Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Richard Kuo <rkuo@codeaurora.org>

arch/hexagon/include/asm/io.h

@@ -189,6 +189,8 @@ static inline void writel(u32 data, volatile void __iomem *addr)
 #define writew_relaxed __raw_writew
 #define writel_relaxed __raw_writel
 
+#define mmiowb()
+
 /*
  * Need an mtype somewhere in here, for cache type deals?
  * This is probably too long for an inline.