uboot-mediatek: update to 2021.04-rc3 with MediaTek's patches

MediaTek published their current U-Boot patchset on github:
https://github.com/mtk-openwrt/u-boot/commits/mtksoc

Import the platform patches from there (`00-mtk-*.patch`), arrange,
them nicely, drop no longer needed local patches and rebase on top of
U-Boot 2021.04-rc3.

Tested and works well on Linksys E8450 (snand-1ddr) as well as
Bananapi BPi-R64 (sdmmc-2ddr, emmc-2ddr).

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

package/boot/uboot-mediatek/patches/100-scripts-remove-dependency-on-swig.patch

@@ -0,0 +1,24 @@
+From b137ca16b54c67d76714ea5a0138741959b0dc29 Mon Sep 17 00:00:00 2001
+From: David Bauer <mail@david-bauer.net>
+Date: Mon, 13 Jul 2020 23:37:37 +0200
+Subject: [PATCH] scripts: remove dependency on swig
+
+Don't build the libfdt tool, as it has a dependency on swig (which
+OpenWrt does not ship).
+
+This requires more hacks, as of-platdata generation does not work
+without it.
+
+Signed-off-by: David Bauer <mail@david-bauer.net>
+---
+ scripts/dtc/Makefile | 2 --
+ 1 file changed, 2 deletions(-)
+
+--- a/scripts/dtc/Makefile
++++ b/scripts/dtc/Makefile
+@@ -18,5 +18,3 @@ HOSTCFLAGS_dtc-parser.tab.o := -I$(src)
+ # dependencies on generated files need to be listed explicitly
+ $(obj)/dtc-lexer.lex.o: $(obj)/dtc-parser.tab.h
+ 
+-# Added for U-Boot
+-subdir-$(CONFIG_PYLIBFDT) += pylibfdt