nocturne: Only advertise Rp-1.5A for sourcing.

BUG=b:117566802
BRANCH=firmware-nocturne-10984.B
TEST=Flash nocturne; verify that 5V/1.5A SourceCaps are sent. Verify
that Rp1.5A is advertised.

Change-Id: Ibc5a45fb15ed5f50ac8609985c4de764c2e59503
Signed-off-by: Aseda Aboagye <aaboagye@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/1274018
Reviewed-by: Benson Leung <bleung@chromium.org>
Commit-Queue: Aseda Aboagye <aaboagye@chromium.org>
Tested-by: Aseda Aboagye <aaboagye@chromium.org>
(cherry picked from commit d118ba10a0d5dfae118e85a898f2b467b41e79ff)
Reviewed-on: https://chromium-review.googlesource.com/1274906
Commit-Ready: Aseda Aboagye <aaboagye@chromium.org>
Reviewed-by: Aseda Aboagye <aaboagye@chromium.org>