Add flash support for SiFive's Freedom E platforms

Valgrind and Clang Static Analyzer have no complaints about this change.

Change-Id: I7757615ec52448372bdc57729cdf97c7016d97e8
Signed-off-by: Tim Newsome <tim@sifive.com>
Reviewed-on: http://openocd.zylin.com/4656


Tested-by: jenkins
Reviewed-by: Philipp Guehring <pg@futureware.at>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>