pwm: th1520: Use module_pwm_platform_driver! macro

The `pwm_th1520` Rust driver calls C functions from the `PWM` namespace,
triggering `modpost` warnings due to missing namespace import
declarations in its `.modinfo` section.

Fix these warnings and simplify the module declaration by switching from
the generic `kernel::module_platform_driver!` macro to the newly
introduced PWM-specific `kernel::module_pwm_platform_driver!` macro.
The new macro automatically handles the required `imports_ns: ["PWM"]`
declaration.

Signed-off-by: Michal Wilczynski <[e-mail]>
Reviewed-by: Troy Mitchell <[e-mail]>
Reviewed-by: Elle Rhumsaa <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Uwe Kleine-König <[e-mail]>

drivers/pwm/pwm_th1520.rs

···
     }
 }
 
-kernel::module_platform_driver! {
+kernel::module_pwm_platform_driver! {
     type: Th1520PwmPlatformDriver,
     name: "pwm-th1520",
     authors: ["Michal Wilczynski <[e-mail]>"],