powerpc/boot: Add mfdcrx

Needed for currituck support.
Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Josh Boyer <jwboyer@gmail.com>

arch/powerpc/boot/dcr.h

@@ -9,6 +9,12 @@
 	})
 #define mtdcr(rn, val) \
 	asm volatile("mtdcr %0,%1" : : "i"(rn), "r"(val))
+#define mfdcrx(rn) \
+	({	\
+		unsigned long rval; \
+		asm volatile("mfdcrx %0,%1" : "=r"(rval) : "r"(rn)); \
+		rval; \
+	})
 
 /* 440GP/440GX SDRAM controller DCRs */
 #define DCRN_SDRAM0_CFGADDR				0x010