mmio: provide basic verbosity

The verbosity must be enabled at two levels: at compile time
via an enum that switches the availability of verbosity code
and via two members of 'Mmio' named '_read_verbose' and
'_write_verbose'. The latter are initialized to 0 (change this to
enable verbosity globally) and can be used to locally enable
(or disable) verbosity in deriving classes.

Ref #753
ahci & exynos5: enable SSDs like Samsung 840 Pro

* Retry debouncing first with a higher trial time and if this also doesn't
  work with lower link speed additionaly.
* Ignore DevSlp feature because it isn't needed anyway as far as i can see.
* Relax some restrictions according the feedback of the drive as far as it
  seem to have no effect in Linux too

Fix #753
ahci & exynos5: try to recover from NCQ errors

Fix little bugs in scatter/gather-list setup, FIS creation, and
command-slot creation of Exynos5 AHCI driver.