[PATCH] drm: xlnx: add <linux/io.h> for readl/writel

[Randy Dunlap <rdunlap@xxxxxxxxxxxxx>]

Add a header file to prevent build errors:

../drivers/gpu/drm/xlnx/zynqmp_dp.c: In function ‘zynqmp_dp_write’:
../drivers/gpu/drm/xlnx/zynqmp_dp.c:335:9: error: implicit declaration of function ‘writel’ [-Werror=implicit-function-declaration]
  335 |         writel(val, dp->iomem + offset);
../drivers/gpu/drm/xlnx/zynqmp_dp.c: In function ‘zynqmp_dp_read’:
../drivers/gpu/drm/xlnx/zynqmp_dp.c:340:16: error: implicit declaration of function ‘readl’ [-Werror=implicit-function-declaration]
  340 |         return readl(dp->iomem + offset);

Fixes: d76271d22694 ("drm: xlnx: DRM/KMS driver for Xilinx ZynqMP DisplayPort Subsystem")
Signed-off-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Cc: Hyun Kwon <hyun.kwon@xxxxxxxxxx>
Cc: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
Cc: "Ville Syrjälä" <ville.syrjala@xxxxxxxxxxxxxxx>
Cc: David Airlie <airlied@xxxxxxxx>
Cc: Daniel Vetter <daniel@xxxxxxxx>
Cc: Michal Simek <michal.simek@xxxxxxxxxx>
Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
---
 drivers/gpu/drm/xlnx/zynqmp_dp.c |    1 +
 1 file changed, 1 insertion(+)

--- a/drivers/gpu/drm/xlnx/zynqmp_dp.c
+++ b/drivers/gpu/drm/xlnx/zynqmp_dp.c
@@ -25,6 +25,7 @@
 #include <linux/clk.h>
 #include <linux/delay.h>
 #include <linux/device.h>
+#include <linux/io.h>
 #include <linux/module.h>
 #include <linux/platform_device.h>
 #include <linux/pm_runtime.h>