Standard way of getting memory map info

Follow up from my comment on #70 where I argue that adding a standard way of querying info about the memory map would be a good idea.

A quick recap:

• Add an extension to query OpenSBI about the memory map at runtime. It's very flexible, but maybe overkill for the most common situation where it remains fixed.
• Pass info at boot time in an structure generated by the firmware: the simplest to implement solution (for both OpenSBI and the payload), but don't allow modifications to the memory map (although after boot, the OS should be able to manage it by itself)
• Do nothing, and rely on info currently available (on almost all platforms the Device Tree, but as a standard I don't know if we can be sure this is going to be true for all platforms which OpenSBI will support). This leads to parsing at certain level platform-specific structures, but maybe it should be that way.