xtensa: add missing isync to the cpu_reset TLB code

ITLB entry modifications must be followed by the isync instruction
before the new entries are possibly used. cpu_reset lacks one isync
between ITLB way 6 initialization and jump to the identity mapping.
Add missing isync to xtensa cpu_reset.

Cc: stable@vger.kernel.org
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>