add SIOplus i/o scheduler

Signed-off-by: Dan Trifan <jimsth@gmail.com>
block/sioplus: sync_read_expire = 312.5ms, writes_starved = 2

Signed-off-by: Dan Trifan <jimsth@gmail.com>
defconfig: enable SIOplus iosched

Signed-off-by: Dan Trifan <jimsth@gmail.com>