Use PTR constant instead of deprecated ptr() function on registers and peripherals

Command: crate build

Full warning:

warning: use of deprecated associated function `cortex_m::peripheral::MPU::ptr`: Use the associated constant `PTR` instead
   --> board-support/src/lib.rs:140:26
    |
140 |         let mpu = &*MPU::ptr();
    |                          ^^^
    |
    = note: `#[warn(deprecated)]` on by default

warning: `nw-board-support` (lib) generated 1 warning

@willemml this seems to be fixed on edf1c4f615f424c4f111ef8ee557631922a4d2bd

I don't know why this happens to me

[willemml]

You might be using nightly rust or a newer version, I don't know why it only happens sometimes, if/when I work on this again I'll check the code for use of deprecated functions.