Vlv2TbltDevicePkg/PlatformPei: Add boot mode detection

Add boot mode detection back into PlatformPei that was
inadvertently removed in the following commit:

https://github.com/tianocore/edk2-platforms/commit/d6211390793fbd0a89b14001c43e0ef942c85425



Boot mode detection at this point is required to detect
the boot mode of BOOT_ON_FLASH_UPDATE that is required
to detect, coalesce, and process UEFI Capsules.

Cc: Zailiang Sun <zailiang.sun@intel.com>
Cc: Yi Qian <yi.qian@intel.com>
Cc: Gary Lin <glin@suse.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Zailiang Sun <zailiang.sun@intel.com>
Tested-by: Gary Lin <glin@suse.com>