ARC: Remove mentions of maintainer mode in readme

Upstream OpenOCD doesn't need option --enable-maintainer-mode anymore.
This patch updates our readme, it removes mention of this options.

Signed-off-by: Anton Kolesov <anton.kolesov@synopsys.com>
ARC: Add debug output to read memory function

Signed-off-by: Anton Kolesov <anton.kolesov@synopsys.com>
ARC: Remove print core/aux regs commands

OpenOCD already has command "reg" to read/write registers. It doesn't
differ core and aux registers though. It also print !exist registers,
but I think this one is a bug. Also no other arch seems to implement
command like this.

Signed-off-by: Anton Kolesov <anton.kolesov@synopsys.com>
ARC: Add support for GDB XML target description

Code to generate XML files for GDB is already in upstream OpenOCD. We just need
to configure out registers properly.

This patch introduces several important changes:
1. JTAG layer now can write arbitrary sets of core register, just like
with AUX register. Now there is no requirement that registers are in
uninterrupted sequence or that function has to be called multiple times.
2. Restore context now restores only dirty registers
3. Save context read only common registers: core, PC and STATUS32.
4. ARC fully support GDB XML target descriptions: registers can be
disabled.
5. OpenOCD now can read BCR to decide which register exist on the fly.
This however is disabled by default, to preserve compatibility with
current GDB. It can be switched on by option "arc gdb-compatibility-mode
false".

I should have made this into several patches, but changes for different
features have been intermixed, so there it would be too difficult to
separate them into logically self-contained patches.

Signed-off-by: Anton Kolesov <anton.kolesov@synopsys.com>