BeagleBoardPkg: Use SerialDxe in MdeModulePkg instead of EmbeddedPkg

BeagleBoardPkg: Use SerialDxe in MdeModulePkg instead of EmbeddedPkg

It is also to remove the reference to TemplateSerialPortExtLib in
EmbeddedPkg.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18970 6f19259b-4bc3-4df7-8a09-765794883524