rdinfra: add a new platform folder 'rdinfra' for all RD infra platforms

The new 'RD' infrastructure/enterprise platforms can reuse all the tests
under the SGI platforms. So add initial set of test build scripts that
reuse all the build scripts for the SGI platform.

Change-Id: If4a406e66e92968f81643c1943e078bf0c64012c
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>