Maintainers.txt: Add maintainer for CSM components in OvmfPkg

Maintainers.txt: Add maintainer for CSM components in OvmfPkg

According to the discussion at:

https://edk2.groups.io/g/devel/topic/31627718#40659
(Request by Laszlo to find proper maintainer for to-be-duplicated CSM
components in OvmfPkg.)

and:

https://edk2.groups.io/g/devel/topic/31682287#40996
(Confirmation with David Woodhouse as the maintainer for the
to-be-duplicated CSM components in OvmfPkg.)

This commit will add David Woodhouse as the maintainer for the CSM
modules under OvmfPkg.

Please note that the duplication of the required CSM modules from
framework packages to OvmfPkg will be done in subsequent commits.

Cc: Ray Ni <ray.ni@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Hao A Wu <hao.a.wu@intel.com>
Reviewed-by: David Woodhouse <dwmw2@infradead.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>