mips: mtmips: add SPL support

This patch adds SPL support for mtmips platform. The lowlevel architecture
is split into SPL and the rest parts are built into a memory loadable
u-boot image. Optional SPL_DM and OF_CONTROL are also supported.

The increment of size is very small (< 10 KiB) if SPL_DM and OF_CONTROL are
not enabled and the memory bootable u-boot (u-boot.img) is generated
automatically so there is not need to add a separate config for it.

A lzma compressed payload (u-boot-lzma.img) is also generated and it will
be combined with u-boot-spl.bin to form the unified ROM bootable binary
u-boot-mtmips.bin.

A spl loader is added to support uncompress the payload.

Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>

arch/mips/dts/mt7628-u-boot.dtsi

@@ -0,0 +1,38 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2020 MediaTek Inc.
+ *
+ * Author: Weijie Gao <weijie.gao@mediatek.com>
+ */
+
+&palmbus {
+	u-boot,dm-pre-reloc;
+};
+
+&reboot {
+	u-boot,dm-pre-reloc;
+};
+
+&clkctrl {
+	u-boot,dm-pre-reloc;
+};
+
+&rstctrl {
+	u-boot,dm-pre-reloc;
+};
+
+&pinctrl {
+	u-boot,dm-pre-reloc;
+};
+
+&uart0 {
+	u-boot,dm-pre-reloc;
+};
+
+&uart1 {
+	u-boot,dm-pre-reloc;
+};
+
+&uart2 {
+	u-boot,dm-pre-reloc;
+};

arch/mips/dts/mt7628a.dtsi

@@ -33,7 +33,7 @@
 		#clock-cells = <0>;
 	};
 
-	palmbus@10000000 {
+	palmbus: palmbus@10000000 {
 		compatible = "palmbus", "simple-bus";
 		reg = <0x10000000 0x200000>;
 		ranges = <0x0 0x10000000 0x1FFFFF>;