Adds libopencm3/cm3/assert.h header that provides assertion check macros
similar to those provided by the standard C library.
Thanks to Nicolas Schodet for help.
file having its own group module rather than using @file.
No code changes except for the following:
gpio: Added function to map the eventout signal plus two Remap functions
dma: Prevent changing base addresses while channel enabled (see datasheet)
pwr: Added pwr.c (new file)
timer: Removed the last function that I introduced recently; there is already
an equivalent function present. Changed some parameter names for consistency.
