staging: fsl-mc: use generic memory barriers

No need to use arch-specific memory barriers; switch to using generic
ones. The rmb()s were useless so drop them.

Signed-off-by: Laurentiu Tudor <laurentiu.tudor@nxp.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>