Maintainers.txt: split out section "OvmfPkg: CSM modules"

Maintainers.txt: split out section "OvmfPkg: CSM modules"

We ask contributors to copy David on patches that affect "CSM modules"
under OvmfPkg. With the "GetMaintainer.py" script being introduced later,
express this expectation in "Maintainers.txt" in machine-readable format.

Cc: Andrew Fish <afish@apple.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: David Woodhouse <dwmw2@infradead.org>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>