configs/csky_gx6605s: bump kernel to 5.0 and use buildroot toolchain

configs/csky_gx6605s: bump kernel to 5.0 and use buildroot toolchain

The most important change is to use the toolchain compiled by
buildroot itself. We also bump kernel to 5.0 with kernel.org.

Gx6605s' PHYS_OFFSET if 0x10000000 and we make qemu and gx6605s the
same to ease maintaince. This PHYS_OFFSET is also OK for 610 qemu.

In this patch we add gx6605s.dts in board/csky, because linux-5.0
doesn't contain gx6605s.dts in its tree.

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>