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 <[e-mail]>
Signed-off-by: Richard Kuo <[e-mail]>

arch/hexagon/include/asm/io.h

···
 #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.