alpha: io: reorder barriers to guarantee writeX() and iowriteX() ordering

memory-barriers.txt has been updated with the following requirement.

"When using writel(), a prior wmb() is not needed to guarantee that the
cache coherent memory writes have completed before writing to the MMIO
region."

Current writeX() and iowriteX() implementations on alpha are not
satisfying this requirement as the barrier is after the register write.

Move mb() in writeX() and iowriteX() functions to guarantee that HW
observes memory changes before performing register operations.

Signed-off-by: Sinan Kaya <okaya@codeaurora.org>
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Matt Turner <mattst88@gmail.com>