Support later revisions of the Foundation FVP

The code in the FVP port which checks the platform type and
revision information in the SYS_ID register strictly supported
only the first revision of the Base and Foundation FVPs.

The current check also does not reflect the fact that the
board revision field is 'local' to the board type (HBI field).

Support for a new Foundation model is required now, and the
checking code is relaxed to allow execution (with a diagnostic)
on unrecognised revisions of the Base and Foundation FVP.

Change-Id: I7cd3519dfb56954aafe5f52ce1fcea0ee257ba9f
Merge pull request #155 from athoelke/at/support-foundation-v2.1

Support later revisions of the Foundation FVP