powerpc/lib: fix unsafe register handling in wait_ticks

If watchdog is enabled, the arch/powerpc/lib/ticks.S::wait_ticks() function
calls the function specified by the WATCHDOG_RESET macro.
The wait_ticks function depends on the registers r0, r6 and r7 being
preserved however that is not guaranteed, e.g. if the reset function is a
C function this will probably overwrite r0 and cause an endless loop.

The following patch changes to using r14+r15 instead of r6+r7 (to resemble
what would have been generated by a C compiler) and saves all necessary
registers on the stack.

The patch has been tested on a custom MPC5125 based machine using the 512x
powerpc architecture.

Signed-off-by: Mats Karrman <mats.karrman@tritech.se>
Cc: Wolfgang Denk <wd@denx.de>
Acked-by: Joakim Tjernlund <joakim.tjernlund@transmode.se>
Tested-by: Stefan Roese <sr@denx.de>

arch/powerpc/lib/ticks.S

@@ -50,19 +50,24 @@ wait_ticks:
 	stwu	r1, -16(r1)
 	mflr	r0		/* save link register */
 	stw	r0, 20(r1)	/* Use r0 or GDB will be unhappy */
-	mr	r7, r3		/* save tick count */
+	stw	r14, 12(r1)	/* save used registers */
+	stw	r15, 8(r1)
+	mr	r14, r3		/* save tick count */
 	bl	get_ticks	/* Get start time */
 
 	/* Calculate end time */
-	addc    r7, r4, r7	/* Compute end time lower */
-	addze	r6, r3		/*     and end time upper */
+	addc    r14, r4, r14	/* Compute end time lower */
+	addze	r15, r3		/*     and end time upper */
 
 	WATCHDOG_RESET		/* Trigger watchdog, if needed */
 1:	bl	get_ticks	/* Get current time */
-	subfc	r4, r4, r7	/* Subtract current time from end time */
-	subfe.	r3, r3, r6
+	subfc	r4, r4, r14	/* Subtract current time from end time */
+	subfe.	r3, r3, r15
 	bge	1b		/* Loop until time expired */
 
-	mtlr	r0		/* restore link register */
+	lwz     r15, 8(r1)	/* restore saved registers */
+	lwz     r14, 12(r1)
+	lwz     r0, 20(r1)
 	addi	r1,r1,16
+	mtlr	r0
 	blr