mxs_gpio: correctly use the GPIO API

mxs_gpio: correctly use the GPIO API

The GPIO API expects a linear GPIO number as parameter to the gpio_*()
functions. The current implementation of the mxs_gpio driver operates on
iomux_cfg_t cookies instead. Therefore the 'gpio' command cannot be
used with this driver.
Change the driver to implement the correct API and introduce some
checks to catch users that still use the old semantics.
1. assert the most sigificant bit in the iomux_cfg_t pad definitions,
   so that using such a cookie in place of a GPIO number will
   decisively generate an invalid GPIO number and flag an error at runtime.
2. introduce a compile switch CONFIG_MXS_IOMUX_COMPILE_CHECK to make
   the iomux_cfg_t cookie a 64 bit variable, so that passing an
   iomux_cfg_t value to a gpio_*() function will generate a compile
   time error.