xtensa: nommu fixes

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
xtensa: ISS: add BLOCK dependency to BLK_DEV_SIMDISK

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
xtensa: provide generic asm/rtc.h

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
xtensa: drop non-existent THREADPTR support from FSF core config

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