selftests/powerpc: Fix matrix multiply assist test

The ISA states: "when ACC[i] contains defined data, the contents of VSRs
4×i to 4×i+3 are undefined until either a VSX Move From ACC instruction
is used to copy the contents of ACC[i] to VSRs 4×i to 4×i+3 or some other
instruction directly writes to one of these VSRs." We aren't doing this.

This test only works on Power10 because the hardware implementation
happens to map ACC0 to VSRs 0-3, but will fail on any other implementation
that doesn't do this. So add xxmfacc between writing to the accumulator
and accessing the VSRs.

Fixes: 3527e1ab9a ("selftests/powerpc: Add matrix multiply assist (MMA) test")
Signed-off-by: Rashmica Gupta <rashmica@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220617043935.428083-1-rashmica@linux.ibm.com

tools/testing/selftests/powerpc/math/mma.S

@@ -20,6 +20,9 @@ test_mma:
 	/* xvi16ger2s */
 	.long	0xec042958
 
+	/* Deprime the accumulator - xxmfacc 0 */
+	.long 0x7c000162
+
 	/* Store result in image passed in r5 */
 	stxvw4x	0,0,5
 	addi	5,5,16