x86: Update x86-pinctrl driver device-tree-bindings doc

This updates the device-tree-bindings doc for x86-pinctrl driver:

 - clarify "gpio-offset" is required only when "mode-gpio" is set
 - correct property name "pull-strength"
 - use tab instead of space at several places

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>