powerpc/64: Allow for THREAD_SIZE > 16k

Fix an assembler error when the THREAD_SIZE is greater than 16k.
Signed-off-by: NHamish Martin <hamish.martin@alliedtelesis.co.nz>
Reviewed-by: NChris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/head_64.S

@@ -949,7 +949,8 @@ start_here_multiplatform:
 	LOAD_REG_ADDR(r3,init_thread_union)
 
 	/* set up a stack pointer */
-	addi	r1,r3,THREAD_SIZE
+	LOAD_REG_IMMEDIATE(r1,THREAD_SIZE)
+	add	r1,r3,r1
 	li	r0,0
 	stdu	r0,-STACK_FRAME_OVERHEAD(r1)