fix(spi): take register block pointer via ptr() instead of regs()

[JurajSadel]

rust-lang/rust#160012 now makes const evaluation check the validity of a reference at the point it is created. A MMIO address is an integer with no provenance, so the reference is not dereferenceable and const evaluation now fails with:

    error[E0080]: reference not dereferenceable: reference must be dereferenceable for 244 bytes, but got 0x60024000[noalloc] which is a dangling pointer (it has no provenance)
  Error:    --> /home/runner/work/esp-hal/esp-hal/esp-hal/src/spi/master/low_level/mod.rs:975:37
      |
  975 |                     register_block: crate::peripherals::$peri::regs(),
      |                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `<peripherals::SPI2<'_> as spi::master::low_level::Instance>::parts::INFO` failed inside this call
      |
  note: inside `peripherals::SPI2::<'_>::regs::<'_>`
     --> /home/runner/work/esp-hal/esp-hal/esp-hal/src/peripherals/mod.rs:162:26
      |
  162 |                   unsafe { &*Self::PTR }
      |                            ^^^^^^^^^^^ the failure occurred here
  ...
  175 | / for_each_peripheral! {
  176 | |     // Define stable peripheral singletons
  177 | |     (@peri_type $(#[$meta:meta])* $name:ident <= $from_pac:tt $interrupts:tt) => {
  178 | |         create_peripheral!( $(#[$meta])* $name <= $from_pac $interrupts);
  ...   |
  277 | |     };
  278 | | }

closes #6020

[RalfJung]

FWIW, doing this on a pointer without provenance is Undefined Behavior both in const-eval and at runtime (except if Target is a zero-sized type). Similarly, writing to and reading from a pointer without provenance is UB (except if the read/write is volatile, or zero-sized).

[bugadani]

We are aware, but the underlying PAC does the same and until std2rust fixes is, we really don't have a good solution.