Maintainers.txt: Add Isaac Oram to common Intel packages

Isaac Oram is actively maintaining Platform/Intel and Silicon/Intel
common packages.

Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Signed-off-by: Isaac Oram <isaac.w.oram@intel.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>