PMU: Add support for deep power-down mode

Currently all low-power modes are supported, except deep power-down mode. The reasons for this are entirely practical: My current setup provides the microcontroller with a voltage that is too high for the deep power-down mode to work (see LPC82x Errata), and I'm currently using the WAKEUP pin for something else.

Still, it would be nice to have support for that mode.