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

drivers/spi/spi-ppc4xx.c

@@ -34,7 +34,7 @@
 #include <linux/spi/spi.h>
 #include <linux/spi/spi_bitbang.h>
 
-#include <asm/io.h>
+#include <linux/io.h>
 #include <asm/dcr.h>
 #include <asm/dcr-regs.h>