ARM: io: ensure inb/outb() et.al. are properly ordered on ARMv6+

Ensure that the ISA/PCI IO space accessors are properly ordered on
ARMv6+ architectures.  These should always be ordered with respect to
all other accesses.

This also fixes __iormb() and __iowmb() not being visible to ioread/
iowrite if a platform defines its own MMIO accessors.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>