arm: add back ldr_dpren macro that was lost.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/arm/asm.S

@@ -119,6 +119,12 @@ T       sub             \rn, \rn, \rm
 T       ldr             \rt, [\rn]
 .endm
 
+.macro  ldr_dpren       rt,  rn,  rm:vararg
+A       ldr             \rt, [\rn, -\rm]
+T       sub             \rt, \rn, \rm
+T       ldr             \rt, [\rt]
+.endm
+
 .macro  ldr_post        rt,  rn,  rm:vararg
 A       ldr             \rt, [\rn], \rm
 T       ldr             \rt, [\rn]