tools: docker: Bump up QEMU version to 6.1.0

tools: docker: Bump up QEMU version to 6.1.0

At present U-Boot CI testing is still using QEMU 4.2.0 which is
pretty old. Let's bump up to QEMU 6.1.0.

ninja-build is added as the prerequisite required by QEMU 6.1.0.

Note there is a bug in QEMU 6.1.0 Xilinx Zynq UART emulation codes.
A quick fix [1] was posted on QEMU mailing list but it it too late
for 6.1.0 release. Let's manually apply the bug fix on top of the
v6.1.0 release tag at the time being.

[1] http://patchwork.ozlabs.org/project/qemu-devel/patch/20210823020813.25192-2-bmeng.cn@gmail.com/

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>