overlays/xtensa_lx106: fix gdb privileged registers definition

xtensa gdb currently cannot query/set privilegad registers. It makes
some sense when gdb is used natively in linux, but not when it's used
with gdbstub on native target (or on QEMU). Mark all special registers
as non-privileged to allow gdb to work with QEMU.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>