powerpc: Drop include of linux/io.h from asm/io.h

Currently powerpc's asm/io.h includes linux/io.h, and linux/io.h
includes asm/io.h.

This can cause problems because depending on which is included first the
order of definitions between the two files will change.

The include of linux/io.h was added back in 2008 in commit b41e5fff
("[POWERPC] devres: Add devm_ioremap_prot()"). It's not entirely clear
it was needed then, but devm_ioremap_prot() has since been removed
entirely as unused, in dedd24a1

 ("powerpc: Remove unused
devm_ioremap_prot()").

So it seems to be unnecessary and can potentially cause problems, so
remove the include of linux/io.h from asm/io.h

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>