Maintainers.txt: add DynamicTablesPkg

Maintainers.txt: add DynamicTablesPkg

DynamicTablesPkg has been in edk2-staging[1] for some time now, and it is
time for it to move into the main tree.
[1] https://github.com/tianocore/edk2-staging/tree/dynamictables

Add Evan and Sami as maintainers of the new package, and let them bring
it in themselves.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Evan Lloyd <evan.lloyd@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>