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>