arch/arm/rp23xx: Fix PWM registers access

[shtirlic]

Fix offset alignment and correct the PWM enable register name

Summary

Current PWM on rp23xx implementation is broken this pull fixes this.

Impact

Working PWM

Testing

Tested on pico2 with single channel and multichannel (2) setup via pwm app.

[nuttxpr]

[Experimental Bot, please feedback here]

No, this PR does not fully meet the NuttX requirements. While it provides some information, it lacks crucial details required for proper review. Here's a breakdown:

• Summary: Too vague. Needs to specify why the current implementation is broken (what's the bug?), what offset alignment is being fixed, and which register name is incorrect. It mentions a functional part (PWM on rp23xx), but not clearly enough. No related issue references are provided.
• Impact: Simply stating "Working PWM" isn't helpful. It needs to explicitly answer all the impact questions (user, build, hardware, documentation, security, compatibility) with "YES/NO" and provide descriptions where necessary. Even if the answer is "NO", it's good practice to explicitly state it.
• Testing: Insufficient. Needs to specify the host OS, CPU, and compiler version used for building. The target information is slightly better, but could be more precise (e.g., "pico2 with xyz configuration"). The biggest issue is the lack of actual logs. Empty code blocks are useless. Provide real logs demonstrating the issue before the change and the corrected behavior after the change.

In short, the PR needs more detail and actual testing logs to be considered complete.