ARM: tegra: acer-a500: Add atmel,wakeup-method property

Acer A500 uses Atmel Maxtouch 1386 touchscreen controller. This controller
has WAKE line which could be connected to I2C clock lane, dedicated GPIO
or fixed to HIGH level. Controller wakes up from a deep sleep when WAKE
line is asserted low. Acer A500 has WAKE line connected to I2C clock and
Linux device driver doesn't work property without knowing what wakeup
method is used by h/w.

Add atmel,wakeup-method property to the touchscreen node.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
[treding@nvidia.com: use literal to avoid dependency on header file]
Signed-off-by: Thierry Reding <treding@nvidia.com>