target-xtensa: implement MISC SR

The Miscellaneous Special Registers Option provides zero to four scratch
registers within the processor readable and writable by RSR, WSR, and
XSR. These registers are privileged. They may be useful for some
application-specific exception and interrupt processing tasks in the
kernel. The MISC registers are undefined after reset.
See ISA, 4.7.3 for details.

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