Back-port AT91SAM7 GPT driver to 2.6.x-stable

git-svn-id: svn://svn.code.sf.net/p/chibios/svn/branches/stable_2.6.x@6827 35acf78f-673a-0410-8e92-d51de3d6d3f4
Back-port AT91SAM7 GPT driver to 2.6.x-stable

git-svn-id: svn://svn.code.sf.net/p/chibios/svn/branches/stable_2.6.x@6828 35acf78f-673a-0410-8e92-d51de3d6d3f4