drm/etnaviv: don't fail to build on arches without PHYS_OFFSET

Some architecture ports like ARC don't provide the PHYS_OFFSET symbol.
Define it to 0 in that case, which is the most conservative default in
the usage context of the etnaviv driver.
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>

drivers/gpu/drm/etnaviv/etnaviv_gpu.c

@@ -31,6 +31,10 @@
 #include "state_hi.xml.h"
 #include "cmdstream.xml.h"
 
+#ifndef PHYS_OFFSET
+#define PHYS_OFFSET 0
+#endif
+
 static const struct platform_device_id gpu_ids[] = {
 	{ .name = "etnaviv-gpu,2d" },
 	{ },