dt-bindings: i2c: at91: fix example for scl-gpios

The SCL gpio pin used by I2C bus for recovery needs to be configured as
open drain, so fix the binding example accordingly.
In relation with fix c5a28380 ("ARM: dts: at91: Configure I2C SCL
gpio as open drain").

Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Fixes: 19e5cef0 ("dt-bindings: i2c: at91: document optional bus recovery properties")