debug: mipi_stp_decoder: Avoid potential 64bit unaligned access

Decoder was casting uint8_t pointers to uint64_t pointers which could
result in double word instruction which does not support unaligned
access on Cortex-M. Issue was revealed when -O3 optimization was
used instead of -Os. In size optimized version, compiler was
using word load and store instructions which support unaligned
access and issue was not visible.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>