Include Protected Device Drivers

[jovanbulck]

We should provide protected MMIO driver variants for some of the devices under src/dev (e.g., keypad). Driver logic should probably best be split up into a minimal asm MMIO SM and higher-level C functions that are to be included in the client protection domain.

Ideally, client SMs should be able to securely use protected I/O devices by including an <sm_support/sm_devname.h> header file, and providing some configuration variables at link time.