io: Open /dev/mem with O_SYNC for uncached access

io: Open /dev/mem with O_SYNC for uncached access

Only when /dev/mem is opened with O_SYNC the write and *read* is done
uncached. We saw wrong values read out from the hardware without setting
O_SYNC, the busybox devmem tool showed different values, when O_SYNC is
also set for the io tool, it reads out the same values as devmem.
When looking at the drivers/char/mem.c file in the kernel it is behaving
differently based on the O_DSYNC flag.

Signed-off-by: Hauke Mehrtens <hauke.mehrtens@intel.com>
(cherry-picked from 90df0f3e241dc233c3121dfc5e1be937c6c987bc)