drivers: pinctrl: ITE: Add a property configure pin current strength

Add the property of drive-strength to drive a high or low current
selection. If this property is not configured, it is the default
setting. According to the SPEC, the default drive current selection
varies from different pins.
Define the high level 0b: 8mA
           low  level 1b: 4mA or 2mA

Signed-off-by: Tim Lin <tim2.lin@ite.corp-partner.google.com>