brcm63xx/lzma-loader: fix O32 ABI conformance

According to the calling convention of the o32 ABI the caller
function must reserve stack space for $a0-$a3 registers in case
the callee needs to save its arguments.

The assembly code of the loader does not reserve stack space for
these registers thus when the 'loader_main' function needs to save
its arguments, those will be stored in the 'workspace' area instead
of the stack.

Because the workspace area is also used by other part of the code, the
saved register values gets overwritten and this often leads to failed
kernel boots.

Fix the code to reserve stack space for the registers to avoid this
error.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
[noltari: apply the fix for brcm63xx too]
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>

SVN-Revision: 48979

target/linux/brcm63xx/image/lzma-loader/src/head.S

@@ -109,6 +109,9 @@ __bss_check:
 	/* Setup new "C" stack */
 	la	sp, _stack
 
+	/* reserve stack space for a0-a3 registers */
+	subu	sp, 16
+
 	/* jump to the decompressor routine */
 	la	t0, loader_main
 	jr	t0