powerpc/64s: scv entry should set PPR

Kernel entry sets PPR to HMT_MEDIUM by convention. The scv entry
path missed this.

Fixes: 7fa95f9ada ("powerpc/64s: system call support for scv/rfscv instructions")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200825075309.224184-1-npiggin@gmail.com

arch/powerpc/kernel/entry_64.S

@@ -113,6 +113,10 @@ END_FTR_SECTION_IFSET(CPU_FTR_TM)
 	ld	r11,exception_marker@toc(r2)
 	std	r11,-16(r10)		/* "regshere" marker */
 
+BEGIN_FTR_SECTION
+	HMT_MEDIUM
+END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
+
 	/*
 	 * RECONCILE_IRQ_STATE without calling trace_hardirqs_off(), which
 	 * would clobber syscall parameters. Also we always enter with IRQs