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.

[iMXRT] add a minimal DMA copy example

While at that, also update the PACs to their latest versions.