xtensa: use register window specific opcodes only when present

xtensa: use register window specific opcodes only when present

xtensa core may be configured without register windows support, don't
use register window specific opcodes in that case. Use window register
specific opcodes to initialize hardware or reset core to a known state
regardless of the chosen ABI.

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