cpu/arm/k60: Clean up LLWU functions.

Rewrote llwu_enable_wakeup_source into two separate functions:
 - llwu_enable_wakeup_module() for handling internal wakeup sources.
 - llwu_set_wakeup_pin() for handling GPIO pin wakeup sources.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: Introduce power_mode_*

Added functions power_mode_vlps, power_mode_lls, power_mode_stop,
power_mode_wait for switching to different power modes.

llwu_sleep() will now use VLPS if allow_deep_sleep is not set, and
LLS mode (like before) when allow_deep_sleep is set.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>