[coreboot] New API to clear DRAM at boot

[Patrick Rudolph]

Hi coreboot folks,
in order to support TEE like Intel TXT it is necessary to be able to
clear all DRAM at boot on request.

As all of the x86 coreboot code is x86_32, it is necessary to make use
of PAE to clear memory.
Please find the attached patch series which proposes an architecure
independed API, PAE enabled memset on x86, and helper functions to
clear all DRAM on Broadwell DE:

https://review.coreboot.org/#/q/project:coreboot+branch:master+topic:securit...

The code can be easily extended to other platforms.
Please comment, test and improve the patchset.

Regards,