pwm: lpss: Fix build failure on PowerPC

An x86 build seems to pull in the linux/io.h include indirectly. On
PowerPC that doesn't happen and the build breaks due to the readl() and
writel() functions not being declared. Fix this by explicitly including
linux/io.h.
Reported-by: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NThierry Reding <thierry.reding@gmail.com>

drivers/pwm/pwm-lpss.c

@@ -13,6 +13,7 @@
  * published by the Free Software Foundation.
  */
 
+#include <linux/io.h>
 #include <linux/kernel.h>
 #include <linux/module.h>