scb.h: Fix register map and bit definitions.

For the change log: This commit breaks backwards compatibility with
0.0.12.  However, it appears that the previous version was incorrect,
so these changes are necessary.

The SCB register map type (scb_reg_map) appears to be adapted from
versions specified by ARM; these include every possible register, and
do not necessarily apply to the STM32, since some registers are
implementation defined, to the extent that the implementation is
allowed not to feature them at all.  Thus, the current definition
appears to be an overreach, since libmaple is STM32-specific.  We
should thus revise it based on ST's PM0056, where the STM32 SCB is
specified.

However, the ST docs appear to be buggy.  In particular, they appear
to contradict requirements made by ARM v7-M ARM with respect to the
debug fault status register (DFSR), which ARM says must exist, but ST
fails to specify (it does leave a DFSR-sized hole in the SCB memory
layout, conveniently located next to some obvious typos which suggest
that the section was not well-proofread).  We defer to ARM and assume
ST just forgot to document the register, and leave the DFSR field in
struct scb_reg_map, since (based on my reading) its absence would be a
silicon bug.

All of the registers appearing in memory addresses higher than that of
SCB_BFAR are not specified by ST, but the v7-M ARM appears to give
some latitude on this to the implementation.  Leave them in the source
text, but put them in an appropriately-commented #if 0 block so that
users who know they're there can yell at us if they find them missing.
In this block, the Auxiliary Feature Register's field was "ADR"
instead of "AFR" in struct scb_reg_map; fix this.

Register bit definitions have been added which are named based on
PM0056 when possible, and on the November 2010 "Derrata 2010_Q3" issue
of the ARM v7-M Architecture Reference Manual, in the case of the
DFSR.

Signed-off-by: Marti Bolivar <mbolivar@leaflabs.com>

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