g: enhance flash INFO api

Flash INFO access API presently is hardcoded to access the endorsement
space. To enable INFO map based rollback protection one needs to be
able to access different areas of the INFO.

The FLASH REGION3 register used by the API is in fact used for NVMEM
flash accesses too, so it is better to leave it alone and use REGION6
for the purpose of accessing of the INFO space.

The API now requires the user to specify flash offset and region size,
the size must be power of 2, the offset must be aligned at size, and
the region must not spill beyond the INFO space address range.

BRANCH=cr50
BUG=b:35774863
TEST=with the rest of the patches applied it is possible to erase the
     base word in the info map segment;
     reading of the endorsement space at startup still works fine.

Change-Id: I008165ee9e11a7b49985d2b30683509cf6699431
Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/447965
Reviewed-by: Nagendra Modadugu <ngm@google.com>
Reviewed-by: Mary Ruthven <mruthven@chromium.org>

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