Input: lpc32xx-key - add clocks property and fix DT binding example

The keypad controller on NXP LPC32xx requires its clock gate to be open,
therefore add description of the requires 'clocks' property.

In addition adjust the example by adding description of required 'clocks'
property and by fixing 'interrupts' property.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>