rtwn: add support for register IO debugging

Add support to read/write the MAC/PHY registers.

Hide it behind RTWN_DEBUG.

This doesn't cover the RF registers as they require a different
IO path, but I haven't yet debugged the RF paths.

Locally tested:

• RTL8192CU, STA
• RTL8188EU, STA
• RTL8812AU / RTL8821AU, STA

Differential Revision: https://reviews.freebsd.org/D48084
Reviewed by: bz