Fix the no. of gpio cells in pxa gpio binding documentation.

The no. of gpio cells for the pxa gpio is actually 2. But is
incorrectly specified as 1, in the binding documentation. From the
driver code, the second cell specifies the standard flags as described
in gpio.txt.
Signed-off-by: Vijay Kumar B. <vijaykumar@zilogic.com>
[fixed subject line]
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>