ShellPkg/Dp: Initialize summary date when run DP

ShellPkg/Dp: Initialize summary date when run DP

Issue:
When run "dp -s" or ("dp -v") command in shell several times,
the summary reuslts are different each time.

The root cause is that the previous global data "SummaryData"
is not cleaned when the dp command is callled next time.
This patch initializes the global data "SummaryData"
when the dp dymanic command is called.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Jaben Carsey <jaben.carsey@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>