powerpc/boot: don't clobber r6 and r7 in epapr boot

We use r6 and r7 for epapr boot, but the current pre-C init will clobber
both of these.

This change does a simple replacement, of r6 -> r12 and r7 -> r13, so
that we hit platform init with these registers intact.
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/boot/crt0.S

@@ -155,29 +155,29 @@ p_base:	mflr	r10		/* r10 now points to runtime addr of p_base */
 	ld	r9,(p_rela-p_base)(r10)
 	add	r9,r9,r10
 
-	li	r7,0
+	li	r13,0
 	li	r8,0
-9:	ld	r6,0(r11)       /* get tag */
-	cmpdi	r6,0
+9:	ld	r12,0(r11)       /* get tag */
+	cmpdi	r12,0
 	beq	12f              /* end of list */
-	cmpdi	r6,RELA
+	cmpdi	r12,RELA
 	bne	10f
-	ld	r7,8(r11)       /* get RELA pointer in r7 */
+	ld	r13,8(r11)       /* get RELA pointer in r13 */
 	b	11f
-10:	addis	r6,r6,(-RELACOUNT)@ha
-	cmpdi	r6,RELACOUNT@l
+10:	addis	r12,r12,(-RELACOUNT)@ha
+	cmpdi	r12,RELACOUNT@l
 	bne	11f
 	ld	r8,8(r11)       /* get RELACOUNT value in r8 */
 11:	addi	r11,r11,16
 	b	9b
 12:
-	cmpdi	r7,0            /* check we have both RELA and RELACOUNT */
+	cmpdi	r13,0            /* check we have both RELA and RELACOUNT */
 	cmpdi	cr1,r8,0
 	beq	3f
 	beq	cr1,3f
 
 	/* Calcuate the runtime offset. */
-	subf	r7,r7,r9
+	subf	r13,r13,r9
 
 	/* Run through the list of relocations and process the
 	 * R_PPC64_RELATIVE ones. */
@@ -185,10 +185,10 @@ p_base:	mflr	r10		/* r10 now points to runtime addr of p_base */
 13:	ld	r0,8(r9)        /* ELF64_R_TYPE(reloc->r_info) */
 	cmpdi	r0,22           /* R_PPC64_RELATIVE */
 	bne	3f
-	ld	r6,0(r9)        /* reloc->r_offset */
+	ld	r12,0(r9)        /* reloc->r_offset */
 	ld	r0,16(r9)       /* reloc->r_addend */
-	add	r0,r0,r7
-	stdx	r0,r7,r6
+	add	r0,r0,r13
+	stdx	r0,r13,r12
 	addi	r9,r9,24
 	bdnz	13b