For all subplaform pins, save the subplatform index as dev->pin and the
actual platform index as dev->phy_pin.
Example:
pin 515 (subplatform pin 3)
dev->pin = 515
dev->phy_pin = 515 - 512 = 3
In this way, methods which get a mraa_gpio_context have a means
to tell whether the pin is on a subplatform or not.
Signed-off-by: Noel Eck <noel.eck@intel.com>