Update uart_16550 to 0.3.2

[Stazer]

Hey,

updating my current project to the 0.15 version of the x86_64 crate introduces compiling issues since the uart_16550 crate version 0.2.18 refers to the x86_64 crate version 0.14. Here is an output from cargo tree:

├── bootloader-x86_64-common v0.11.10
│   ├── bootloader-boot-config v0.11.10
...
│   ├── bootloader_api v0.11.10
...
│   ├── uart_16550 v0.2.19
...
│   │   └── x86_64 v0.14.13
...
│   ├── x86_64 v0.15.2
...

I have updated uart_16550 accordingly. It does not depend on x86_64 anymore with version 0.3.2.

Best Regards

[Freax13]

Thank you for your contribution!

Looks good to me, thanks!