xtensa: ISS: allow simdisk to use high memory buffers

ISS kernel by default has only low memory. But it may be configured to
support high memory and started in a simulator with more than 128M of
RAM. Simdisk driver in such configuration can get IO request with a
high memory page. There may be no TLB entry for that page, only page
table entry. However simulators don't do pagewalking, so such IO request
will fail. Touch IO buffer in the buffer read/write loop so that a TLB
entry is likely there when read or write simcall is invoked.

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