ARM: PSCI: use only r0 and r3 in psci_get_cpu_stack_top()

For psci_get_cpu_stack_top() to be usable in C code, it must adhere to
the ARM calling conventions. Since it could be called when the stack
is still unavailable, and the entry code to linux also expects r1 and
r2 to remain unchanged, stick to r0 and r3.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

arch/arm/cpu/armv7/psci.S

@@ -196,15 +196,15 @@ ENDPROC(psci_cpu_off_common)
 
 @ expects CPU ID in r0 and returns stack top in r0
 ENTRY(psci_get_cpu_stack_top)
-	mov	r5, #0x400			@ 1kB of stack per CPU
-	mul	r0, r0, r5
-
-	ldr	r5, =psci_text_end		@ end of monitor text
-	add	r5, r5, #0x2000			@ Skip two pages
-	lsr	r5, r5, #12			@ Align to start of page
-	lsl	r5, r5, #12
-	sub	r5, r5, #4			@ reserve 1 word for target PC
-	sub	r0, r5, r0			@ here's our stack!
+	mov	r3, #0x400			@ 1kB of stack per CPU
+	mul	r0, r0, r3
+
+	ldr	r3, =psci_text_end		@ end of monitor text
+	add	r3, r3, #0x2000			@ Skip two pages
+	lsr	r3, r3, #12			@ Align to start of page
+	lsl	r3, r3, #12
+	sub	r3, r3, #4			@ reserve 1 word for target PC
+	sub	r0, r3, r0			@ here's our stack!
 
 	bx	lr
 ENDPROC(psci_get_cpu_stack_top)