This makes life easier when calling e.g. `saul_reg_write()` with data
stored in flash.
As now the signatures for reading and writing differ (in that `const`
qualifier only), `saul_notsup()` is split into `saul_write_notsup()`
and `saul_read_notsup()`. However, one is implemented as a symbol alias
of the other, so that ROM consumption remains unchanged.
- renambed to mma8x5x to reflect the versatility
- cleaned up the interface
- fixed value computation for non 12-bit devices of the family
- added default configuration with mma8x5x_params.h
