arc: dts: separate single axs10x.dts file

We want to use the same device tree blobs in both Linux and U-Boot for
ARC boards.

Earlier device tree sources in U-Boot were very simplified and hadn't been
updated for quite a long period of time.

So this commit is the first step on the road to unified device tree blobs.

First of all we re-organize device tree sources for AXS10X boards.
As AXS101 and AXS103 boards consist of AXS10X motherboard and AXC001 and
AXC003 cpu tiles respectively we add corresponding device tree source
files: axs10x_mb.dtsi for motherboard, axc001.dtsi and axc003.dtsi for
cpu tiles and axs101.dts and axs103.dts to represent actual boards.

Also we delete axs10x.dts as it is no longer used.

One more important change - we add timer device to ARC skeleton device
tree sources as both ARC700 and ARCHS cores contain such timer.
We add core_clk nodes to abilis_tb100, nsim, axc001 and axc003 device tree
sources as it is referenced via phandle from timer node in common
skeleton.dtsi file.

Signed-off-by: Vlad Zakharov <vzakhar@synopsys.com>
Reviewed-by: Simon Glass <sjg@chromium.org>