x86: ich6_gpio: Add use-lvl-write-cache for I/O access mode

Add a device-tree property use-lvl-write-cache that will cause
writes to lvl to be cached instead of read from lvl before each
write. This is required on some platforms that have the register
implemented as dual read/write (such as Baytrail).

Prior to this fix the blue USB port on the Minnowboard Max was
unusable since USB_HOST_EN0 was set high then immediately set
low when USB_HOST_EN1 was written.

This also resolves the 'gpio clear | set' command warning like:
  "Warning: value of pin is still 0"

Signed-off-by: George McCollister <george.mccollister@gmail.com>
<rebased on latest origin/master, fixed all baytrail boards>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>