pinctrl: imx21: Remove the driver

Since commit 4b563a06

 ("ARM: imx: Remove imx21 support") the imx21
SoC is no longer supported.

Get rid of its pinctrl driver too, which is now unused.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Shawn Guo <shawnguo@kernel.org>
Link: https://lore.kernel.org/r/20201110190210.29376-1-festevam@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>