MX: Set a common gpio.h for all i.MX

MX: Set a common gpio.h for all i.MX

Each i.MX has its own gpio.h, defining the same structure.
The internal GPIO controller has the same layout
(at least for the register used by u-boot) and can be shared.

Signed-off-by: Stefano Babic <sbabic@denx.de>
Tested-by: Matt Sealey <matt@genesi-usa.com>