Enable IDLE and Deep Powerdown mode.
IDLE is pretty straightforward - insteady of busy waiting, the CPU will
enter an idle state from which it will resume on any event.
Deep Power Down shuts off the entite system except for the battery backup
power domain.
That means the CPU will reset on resume and can be woken by e.g. RTC.
SLEEP and POWERDOWN disable the PLL and the PLL and Flash respectively.
This requires some proper wake-up handling.
Since this turned out to be a major time sink and those modes are never
currently never used in RIOT outside of tests, I left this as an exercise
for a future reader.
Rational: the periph_common module is required by (most) other periph drivers
and also during startup of the CPU/MCU to run periph_init. The latter is only
required if other periph drivers are used, hence periph_common should be a
depency of periph_* modules and *not* of the CPU/MCU. This PR fixes that
by making periph_common a depency of periph_* and removing the explicit
include in the CPU/MCU implementation.
Rationale 1: The common part made only sense for (some) NXP ARM7 MCUs,
but was misleading for MCUs like the LPC1768.
Rationale 2: The common part was only used by one specific MCU
implementation - no need to outsource it.
