Maintainer.txt: add myself as reviewer for bhyve's OvmfPkg

Signed-off-by: Corvin Köhne <corvink@FreeBSD.org>
Reviewed-by: Rebecca Cran <rebecca@bsdio.com>
Reviewed-by: Peter Grehan <grehan@freebsd.org>
Cc: Andrew Fish <afish@apple.com>
Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>