armv8: fsl-layerscape: Support loading 32-bit OS with PSCI enabled

As PSCI and secure monitor firmware framework are enabled, this patch is
to support loading 32-bit OS in such case. The default target exception
level returned to U-Boot is EL2, so the corresponding work to switch to
AArch32 EL2 and jump to 32-bit OS are done in U-Boot and secure firmware
together.

Signed-off-by: Alison Wang <alison.wang@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>
diff --git a/arch/arm/cpu/armv8/transition.S b/arch/arm/cpu/armv8/transition.S
index bbccf2b..adb9f35 100644
--- a/arch/arm/cpu/armv8/transition.S
+++ b/arch/arm/cpu/armv8/transition.S
@@ -13,6 +13,14 @@
 ENTRY(armv8_switch_to_el2)
 	switch_el x5, 1f, 0f, 0f
 0:
+	cmp x4, #ES_TO_AARCH64
+	b.eq 2f
+	/*
+	 * When loading 32-bit kernel, it will jump
+	 * to secure firmware again, and never return.
+	 */
+	bl armv8_el2_to_aarch32
+2:
 	/*
 	 * x3 is kernel entry point or switch_to_el1
 	 * if CONFIG_ARMV8_SWITCH_TO_EL1 is defined.
@@ -32,3 +40,7 @@
 	br x3
 1:	armv8_switch_to_el1_m x3, x4, x5
 ENDPROC(armv8_switch_to_el1)
+
+WEAK(armv8_el2_to_aarch32)
+	ret
+ENDPROC(armv8_el2_to_aarch32)