unicore32: pwm: Use module_platform_driver()

Some of the boilerplate code can be eliminated by using this macro. The
driver was previously registered with an arch_initcall(), so technically
this is no longer the same, but when the driver is moved to the PWM
framework, deferred probing will take care of any driver probe ordering
issues.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Tested-by: Qin Rui <qinrui@mprc.pku.edu.cn>
Acked-by: Guan Xuetao <gxt@mprc.pku.edu.cn>