ARM: imx: Remove unused IMX_GPIO_NR() macro

The IMX_GPIO_NR() macro was only used by non-DT i.MX platforms.

As i.MX transitioned to a DT-only platform, get rid of this
unused macro.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>