pinctrl: remove ste u300 driver

The ST-Ericsson U300 platform is getting removed, so this driver is no
longer needed.

Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20210120132045.2127659-6-arnd@kernel.org


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>