Input: atmel-wm97xx - only build for AVR32

Building this driver on ARM/at91 always gives us this error message:

drivers/input/touchscreen/atmel-wm97xx.c:63:2: error: #error Unknown CPU, this driver only supports AT32AP700X CPUs.

Clearly this configuration is not meant to work, so let's just prevent
it in Kconfig. If we ever want to use it on another platform, we should
also pass proper resources for GPIO, IRQ and memory, which are hardcoded
to AT32AP700X at the moment.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>