AVR32: Relocate u-boot to SDRAM

Relocate the u-boot image into SDRAM like everyone else does. This
means that we can handle much larger .data and .bss than we used to.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
diff --git a/cpu/at32ap/entry.S b/cpu/at32ap/entry.S
index b52d798..a6fc688 100644
--- a/cpu/at32ap/entry.S
+++ b/cpu/at32ap/entry.S
@@ -42,8 +42,7 @@
 	 * We're running at interrupt level 3, so we don't need to save
 	 * r8-r12 or lr to the stack.
 	 */
-	mov	r8, lo(timer_overflow)
-	orh	r8, hi(timer_overflow)
+	lda.w	r8, timer_overflow
 	ld.w	r9, r8[0]
 	mov	r10, -1
 	mtsr	SYSREG_COMPARE, r10