pwm: xilinx: Make use of devm_pwmchip_alloc() function

This prepares the pwm-xilinx driver to further changes of the pwm core
outlined in the commit introducing devm_pwmchip_alloc(). There is no
intended semantical change and the driver should behave as before.

Link: https://lore.kernel.org/r/7cbc32771e94103b8c1c817cfdd613d7a2fc01b9.1707900770.git.u.kleine-koenig@pengutronix.de


Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>