gpio: em: Remove obsolete platform data support

Since commit 59032702 ("ARM: shmobile: Remove legacy platform
devices from EMEV2 SoC code"), EMMA Mobile SoCs are only supported in
generic DT-only ARM multi-platform builds.  The driver doesn't need to
use platform data anymore, hence remove platform data configuration.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Tested-by: Niklas Söderlund <niso@kth.se>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>