spi: ppc4xx: include <linux/io.h> instead of <asm/io.h>

Include the more general linux/io.h instead of asm/io.h
as checkpatch suggests.

Signed-off-by: Jay Fang <f.fangjian@huawei.com>
Link: https://lore.kernel.org/r/1620629903-15493-2-git-send-email-f.fangjian@huawei.com


Signed-off-by: Mark Brown <broonie@kernel.org>