ARM: io: fix ioremap_wt() implementation

ioremap_wt() was added by aliasing it to ioremap_nocache(), which is a
device mapping.  Device mappings do not allow unaligned accesses, but
it appears that GCC is able to inline its own memcpy() implementation
which may use such accesses.  The only user of this is pmem, which
uses memcpy() on the region.

Therefore, this is unsafe.  We must implement ioremap_wt() correctly
for ARM, or not at all.

This patch adds a more correct implementation by re-using ioremap_wc()
to provide a normal-memory non-cacheable mapping.

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