Blackfin: debug-mmrs: fix typos with gptimers/mdma/ppi

This code was mostly developed against a BF54x, so some BF537-specific
issues were missed.

The PPI block starts at PPI_CONTROL, not PPI_STATUS (which is the reverse
of the EPPI block).

The MDMA block starts at MDMA_NEXT_DESC_PTR, not MDMA_CONFIG.  Seems the
sim does not catch misreads here so that'll need to get fixed.

The gptimer block is mostly 32bit regs, not 16bit.  Use the gptimer struct
to figure that out rather than hardcoding it locally.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>