cpu/stm32l1: add stop and standby modes, adds pm_layered

[fjmolinas]

Contribution description

This PR adds low power modes STOP and STANDBY for stm32l1 boards. Achieves 100uA on STOP mode. For lower power consumption GPIO must be switched to AIN on startup, this is left for a different PR (2uA with this addition but this is not done automatically in L1 when entering LPM, which L0 does).

Testing procedure

Connect a multimeter or other current measuring device on IDD pin on nucleo-l152re boards, then run:

make BOARD=nucleo-l152re -C tests/periph_pm/ PORT=/dev/ttyACM0 flash term

Unblock using rtc mode 0 and 1 to see effect on current measurement/consumption.

Issues/PRs references

Based on #8403, #7329, #10052

[kaspar030]

Achieves 100mA on STOP mode.

is that milli or microamps?

[fjmolinas]

@kaspar030 uA, sorry for the typo

[aabadie]

Using tests/periph_pm on a nucleo-l152re, I could test this PR and can confirm that low power can be reached with stm32l1. There are still problems with STANDBY mode: the board doesn't recover properly sometimes.

I'll also do some test on an L0 board, since this family is also affected by this change.

[aabadie]

@fjmolinas, this needs rebase, now that #11167 is merged.

[fjmolinas]

@aabadie rebased

[aabadie]

I tested this PR on nucleo-l152re and confirm that low-power can be reached in STOP and STANDBY modes. Values measured in STOP mode are around 60uA, in STANDBY mode the consumption is around 1uA.

Since this PR is also touching the stm32l0, I also tested this PR on nucleo-l073rz. There is a significant improvement in consumption with master thanks to the ultra-low power mode being used.

ACK and go

[vincent-d]

I had #9521 pending for a long time, which also refactored a bit...will need to rebase.