count should be defined as the integer div_round_up of buffer length
and word size, otherwise transferring small buffers will cause a panic
due to underflow:

if we let from = [0u32; 1];

then calling dma::write() will result in:

let count = ((1 / 4 as usize) - 1)
          = 0 - 1 // underflow

Rounding up results in 1 - 1 as expected.

Instead of adding magic constants all over the place, let's just
define DMA MAX_CHUNK_SIZE in a single constant that be referenced by
the various users.

Before writing to MATCH_L/MATCH_H registers, wait for permission to do
so by pollinmg MATCH_WR_RDY bit in CTRL register.

Co-Authored-By: Aurélien Jacobs <aurel@gnuage.org>

add the possibility to document `bind_interrupts` `struct`s

the `bind_interrupts` macro creates a `struct` for the interrupts. it
was so far not possible to document those (except for STM32) and there
was no generic documentation being generated/added either, thus the
`missing_docs` lint was triggered for consumers which enabled it.

with this change it is now possible to manually add a comment on the
`struct` being defined in the macro invocation.

to show that this works one RP example has been modified accordingly.