mmc: rmobile: add a compiler barrier

mmc: rmobile: add a compiler barrier

Building w/ GCC v5.2, the SD card access is broken due to invalid data
in the response command reconstructed at the end of
sh_sdhci_get_response().

Add a memory barrier between the two main steps of this function to
ensure the resp[] table content is consistent before bits reordering.

This fix has been tested Ok on Porter board rev1.0 using v2016.03
release.

Signed-off-by: Yannick Gicquel <yannick.gicquel@iot.bzh>