gpio: ts4800: Add IMX51 dependency

The TS-4800 is an i.MX515 board. Its GPIO driver should only be compiled
for this CPU or for test builds.

Signed-off-by: Julien Grossholtz <julien.grossholtz@savoirfairelinux.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>