timer: cadence: Add bind function to driver

When DT node has pwm-cells property it shouldn't be bind as timer driver
but as PWM driver. That's why make sure that this property is checked.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Link: https://lore.kernel.org/r/434ef195fbedea9f83672a12d1ace0da16e8832e.1634303847.git.michal.simek@xilinx.com