MIPS: lantiq: Directly include linux/of.h in xway/dma.c

This drivers currently obtains linux/of.h by luck and a chain of
bizarre inclusions, which we're about to fix.

Let's include the required file directly.
Signed-off-by: Marc Zyngier <maz@kernel.org>

arch/mips/lantiq/xway/dma.c

@@ -12,6 +12,7 @@
 #include <linux/spinlock.h>
 #include <linux/clk.h>
 #include <linux/err.h>
+#include <linux/of.h>
 
 #include <lantiq_soc.h>
 #include <xway_dma.h>