ARC: SMP: debug: workaround MDB changing debug_select value

MDB debugger may modify debug_select and debug_mask registers
on start, so we can't rely on debug_select reset value.

Let's set correct value on primary CPU without reading initial
value from debug_select.

Internal ID: P10019563-50516

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>