roms/u-boot: Bump ppce500 u-boot to v2021.04 to fix broken pci support

When QEMU originally supported the ppce500 machine back in Jan 2014,
it was created with a 1:1 mapping of PCI bus address. Things seemed
to change rapidly that in Nov 2014 with the following QEMU commits:

commit e6b4e5f479 ("PPC: e500: Move CCSR and MMIO space to upper end of address space")

and

commit cb3778a045 ("PPC: e500 pci host: Add support for ATMUs")

the PCI memory and IO physical address were moved to beyond 4 GiB,
but PCI bus address remained below 4 GiB, hence a non-identity
mapping was created. Unfortunately corresponding U-Boot updates
were missed along with the QEMU changes and the U-Boot QEMU ppce500
PCI support has been broken since then, until this issue was fixed
recently in U-Boot mainline v2021.04 release, specifically by the
following U-Boot series:

http://patchwork.ozlabs.org/project/uboot/list/?series=230985&state=*

The cross-compilation toolchain used to build the U-Boot image is:
https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/x86_64/10.1.0/x86_64-gcc-10.1.0-nolibc-powerpc-linux.tar.xz

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

roms/u-boot

@@ -1 +1 @@
-Subproject commit d3689267f92c5956e09cc7d1baa4700141662bff
+Subproject commit b46dd116ce03e235f2a7d4843c6278e1da44b5e1