Input: atmel_captouch - add dependence on i2c

The newly added Atmal captouch driver fails to build when CONFIG_I2C
is disabled:

drivers/input/misc/atmel_captouch.c: In function 'atmel_read':
drivers/input/misc/atmel_captouch.c:115:8: error: implicit declaration of function 'i2c_transfer' [-Werror=implicit-function-declaration]

This adds an explicit Kconfig dependency to prevent the broken
configuration.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: b06d43f7

 ("Input: add Atmel Captouch Button driver")
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>